import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author AyushKM
 */
public class Main {
   public static void main(String[] args) {
      InputStream inputStream = System.in;
      OutputStream outputStream = System.out;
      InputReader in = new InputReader(inputStream);
      OutputWriter out = new OutputWriter(outputStream);
      BResistorsInParallel solver = new BResistorsInParallel();
      solver.solve(1, in, out);
      out.close();
   }

   static class BResistorsInParallel {
      public void solve(int testNumber, InputReader in, OutputWriter out) {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      }

   }

   static class InputReader {
      private InputStream stream;
      private byte[] buf = new byte[1024];
      private int curChar;
      private int numChars;
      private InputReader.SpaceCharFilter filter;

      public InputReader(InputStream stream) {
         this.stream = stream;
      }

      public int read() {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      }

      public int nextInt() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      }

      public boolean isSpaceChar(int c) {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      }

      public static boolean isWhitespace(int c) {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      }

      public double nextDouble() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      }

      public interface SpaceCharFilter {
         public boolean isSpaceChar(int ch);

      }

   }

   static class OutputWriter {
      private final PrintWriter writer;

      public OutputWriter(OutputStream outputStream) {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      }

      public OutputWriter(Writer writer) {
         this.writer = new PrintWriter(writer);
      }

      public void print(Object... objects) {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      }

      public void println(Object... objects) {
         print(objects);
         writer.println();
      }

      public void close() {
         writer.close();
      }

   }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.OutputStream; OutputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.io.BufferedWriter; BufferedWriter java.io import java.io.Writer; Writer java.io import java.io.OutputStreamWriter; OutputStreamWriter java.io import java.util.InputMismatchException; InputMismatchException java.util import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io public class Main {
   public static void main(String[] args) {
      InputStream inputStream = System.in;
      OutputStream outputStream = System.out;
      InputReader in = new InputReader(inputStream);
      OutputWriter out = new OutputWriter(outputStream);
      BResistorsInParallel solver = new BResistorsInParallel();
      solver.solve(1, in, out);
      out.close();
   }

   static class BResistorsInParallel {
      public void solve(int testNumber, InputReader in, OutputWriter out) {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      }

   }

   static class InputReader {
      private InputStream stream;
      private byte[] buf = new byte[1024];
      private int curChar;
      private int numChars;
      private InputReader.SpaceCharFilter filter;

      public InputReader(InputStream stream) {
         this.stream = stream;
      }

      public int read() {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      }

      public int nextInt() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      }

      public boolean isSpaceChar(int c) {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      }

      public static boolean isWhitespace(int c) {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      }

      public double nextDouble() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      }

      public interface SpaceCharFilter {
         public boolean isSpaceChar(int ch);

      }

   }

   static class OutputWriter {
      private final PrintWriter writer;

      public OutputWriter(OutputStream outputStream) {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      }

      public OutputWriter(Writer writer) {
         this.writer = new PrintWriter(writer);
      }

      public void print(Object... objects) {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      }

      public void println(Object... objects) {
         print(objects);
         writer.println();
      }

      public void close() {
         writer.close();
      }

   }
} Main public static void main(String[] args) {
      InputStream inputStream = System.in;
      OutputStream outputStream = System.out;
      InputReader in = new InputReader(inputStream);
      OutputWriter out = new OutputWriter(outputStream);
      BResistorsInParallel solver = new BResistorsInParallel();
      solver.solve(1, in, out);
      out.close();
   } main {
      InputStream inputStream = System.in;
      OutputStream outputStream = System.out;
      InputReader in = new InputReader(inputStream);
      OutputWriter out = new OutputWriter(outputStream);
      BResistorsInParallel solver = new BResistorsInParallel();
      solver.solve(1, in, out);
      out.close();
   } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out InputReader in = new InputReader(inputStream); in new InputReader(inputStream) OutputWriter out = new OutputWriter(outputStream); out new OutputWriter(outputStream) BResistorsInParallel solver = new BResistorsInParallel(); solver new BResistorsInParallel() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class BResistorsInParallel {
      public void solve(int testNumber, InputReader in, OutputWriter out) {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      }

   } BResistorsInParallel public void solve(int testNumber, InputReader in, OutputWriter out) {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      } solve {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      } int n = in.nextInt(); n in.nextInt() in.nextInt in double a = 0; a 0 for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         } int x = 0; int x = 0; x 0 x < n x n x++ x++ x {
            a += (1.0 / in.nextDouble());
         } {
            a += (1.0 / in.nextDouble());
         } a += (1.0 / in.nextDouble()) a (1.0 / in.nextDouble()) 1.0 in.nextDouble() in.nextDouble in out.println(1 / a) out.println out 1 / a 1 a int testNumber testNumber InputReader in in OutputWriter out out static class InputReader {
      private InputStream stream;
      private byte[] buf = new byte[1024];
      private int curChar;
      private int numChars;
      private InputReader.SpaceCharFilter filter;

      public InputReader(InputStream stream) {
         this.stream = stream;
      }

      public int read() {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      }

      public int nextInt() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      }

      public boolean isSpaceChar(int c) {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      }

      public static boolean isWhitespace(int c) {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      }

      public double nextDouble() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      }

      public interface SpaceCharFilter {
         public boolean isSpaceChar(int ch);

      }

   } InputReader private InputStream stream; stream private byte[] buf = new byte[1024]; buf new byte[1024] 1024 private int curChar; curChar private int numChars; numChars private InputReader.SpaceCharFilter filter; filter public InputReader(InputStream stream) {
         this.stream = stream;
      } InputReader {
         this.stream = stream;
      } this.stream = stream this.stream this this.stream stream InputStream stream stream public int read() {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      } read {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      } if (numChars == -1) {
            throw new InputMismatchException();
         } numChars == -1 numChars -1 1 {
            throw new InputMismatchException();
         } throw new InputMismatchException(); new InputMismatchException() if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         } curChar >= numChars curChar numChars {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         } curChar = 0 curChar 0 try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            } catch (IOException e) {
               throw new InputMismatchException();
            } IOException e {
               throw new InputMismatchException();
            } throw new InputMismatchException(); new InputMismatchException() {
               numChars = stream.read(buf);
            } numChars = stream.read(buf) numChars stream.read(buf) stream.read stream buf if (numChars <= 0) {
               return -1;
            } numChars <= 0 numChars 0 {
               return -1;
            } return -1; -1 1 return buf[curChar++]; buf[curChar++] buf curChar++ curChar public int nextInt() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      } nextInt {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      } int c = read(); c read() read while (isSpaceChar(c)) {
            c = read();
         } isSpaceChar(c) isSpaceChar c {
            c = read();
         } c = read() c read() read int sgn = 1; sgn 1 if (c == '-') {
            sgn = -1;
            c = read();
         } c == '-' c '-' {
            sgn = -1;
            c = read();
         } sgn = -1 sgn -1 1 c = read() c read() read int res = 0; res 0 do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c)); !isSpaceChar(c) isSpaceChar(c) isSpaceChar c {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } if (c < '0' || c > '9') {
               throw new InputMismatchException();
            } c < '0' || c > '9' c < '0' c '0' c > '9' c '9' {
               throw new InputMismatchException();
            } throw new InputMismatchException(); new InputMismatchException() res *= 10 res 10 res += c - '0' res c - '0' c '0' c = read() c read() read return res * sgn; res * sgn res sgn public boolean isSpaceChar(int c) {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      } isSpaceChar {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      } if (filter != null) {
            return filter.isSpaceChar(c);
         } filter != null filter null {
            return filter.isSpaceChar(c);
         } return filter.isSpaceChar(c); filter.isSpaceChar(c) filter.isSpaceChar filter c return isWhitespace(c); isWhitespace(c) isWhitespace c int c c public static boolean isWhitespace(int c) {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      } isWhitespace {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      } return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1; c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1 c == '\t' c == '\r' c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1 c == ' ' c ' ' c == '\n' c '\n' c == '\r' c '\r' c == '\t' c '\t' c == -1 c -1 1 int c c public double nextDouble() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      } nextDouble {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      } int c = read(); c read() read while (isSpaceChar(c)) {
            c = read();
         } isSpaceChar(c) isSpaceChar c {
            c = read();
         } c = read() c read() read int sgn = 1; sgn 1 if (c == '-') {
            sgn = -1;
            c = read();
         } c == '-' c '-' {
            sgn = -1;
            c = read();
         } sgn = -1 sgn -1 1 c = read() c read() read double res = 0; res 0 while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } !isSpaceChar(c) && c != '.' !isSpaceChar(c) isSpaceChar(c) isSpaceChar c c != '.' c '.' {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            } c == 'e' || c == 'E' c == 'e' c 'e' c == 'E' c 'E' {
               return res * Math.pow(10, nextInt());
            } return res * Math.pow(10, nextInt()); res * Math.pow(10, nextInt()) res Math.pow(10, nextInt()) Math.pow Math 10 nextInt() nextInt if (c < '0' || c > '9') {
               throw new InputMismatchException();
            } c < '0' || c > '9' c < '0' c '0' c > '9' c '9' {
               throw new InputMismatchException();
            } throw new InputMismatchException(); new InputMismatchException() res *= 10 res 10 res += c - '0' res c - '0' c '0' c = read() c read() read if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         } c == '.' c '.' {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         } c = read() c read() read double m = 1; m 1 while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            } !isSpaceChar(c) isSpaceChar(c) isSpaceChar c {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            } if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               } c == 'e' || c == 'E' c == 'e' c 'e' c == 'E' c 'E' {
                  return res * Math.pow(10, nextInt());
               } return res * Math.pow(10, nextInt()); res * Math.pow(10, nextInt()) res Math.pow(10, nextInt()) Math.pow Math 10 nextInt() nextInt if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               } c < '0' || c > '9' c < '0' c '0' c > '9' c '9' {
                  throw new InputMismatchException();
               } throw new InputMismatchException(); new InputMismatchException() m /= 10 m 10 res += (c - '0') * m res (c - '0') * m (c - '0') c '0' m c = read() c read() read return res * sgn; res * sgn res sgn public interface SpaceCharFilter {
         public boolean isSpaceChar(int ch);

      } SpaceCharFilter public boolean isSpaceChar(int ch); isSpaceChar int ch ch static class OutputWriter {
      private final PrintWriter writer;

      public OutputWriter(OutputStream outputStream) {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      }

      public OutputWriter(Writer writer) {
         this.writer = new PrintWriter(writer);
      }

      public void print(Object... objects) {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      }

      public void println(Object... objects) {
         print(objects);
         writer.println();
      }

      public void close() {
         writer.close();
      }

   } OutputWriter private final PrintWriter writer; writer public OutputWriter(OutputStream outputStream) {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      } OutputWriter {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      } writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream))) writer new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream))) OutputStream outputStream outputStream public OutputWriter(Writer writer) {
         this.writer = new PrintWriter(writer);
      } OutputWriter {
         this.writer = new PrintWriter(writer);
      } this.writer = new PrintWriter(writer) this.writer this this.writer new PrintWriter(writer) Writer writer writer public void print(Object... objects) {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      } print {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      } for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         } int i = 0; int i = 0; i 0 i < objects.length i objects.length objects objects.length i++ i++ i {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         } {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         } if (i != 0) {
               writer.print(' ');
            } i != 0 i 0 {
               writer.print(' ');
            } writer.print(' ') writer.print writer ' ' writer.print(objects[i]) writer.print writer objects[i] objects i Object... objects objects public void println(Object... objects) {
         print(objects);
         writer.println();
      } println {
         print(objects);
         writer.println();
      } print(objects) print objects writer.println() writer.println writer Object... objects objects public void close() {
         writer.close();
      } close {
         writer.close();
      } writer.close() writer.close writer public class Main {
   public static void main(String[] args) {
      InputStream inputStream = System.in;
      OutputStream outputStream = System.out;
      InputReader in = new InputReader(inputStream);
      OutputWriter out = new OutputWriter(outputStream);
      BResistorsInParallel solver = new BResistorsInParallel();
      solver.solve(1, in, out);
      out.close();
   }

   static class BResistorsInParallel {
      public void solve(int testNumber, InputReader in, OutputWriter out) {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      }

   }

   static class InputReader {
      private InputStream stream;
      private byte[] buf = new byte[1024];
      private int curChar;
      private int numChars;
      private InputReader.SpaceCharFilter filter;

      public InputReader(InputStream stream) {
         this.stream = stream;
      }

      public int read() {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      }

      public int nextInt() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      }

      public boolean isSpaceChar(int c) {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      }

      public static boolean isWhitespace(int c) {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      }

      public double nextDouble() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      }

      public interface SpaceCharFilter {
         public boolean isSpaceChar(int ch);

      }

   }

   static class OutputWriter {
      private final PrintWriter writer;

      public OutputWriter(OutputStream outputStream) {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      }

      public OutputWriter(Writer writer) {
         this.writer = new PrintWriter(writer);
      }

      public void print(Object... objects) {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      }

      public void println(Object... objects) {
         print(objects);
         writer.println();
      }

      public void close() {
         writer.close();
      }

   }
} public class Main {
   public static void main(String[] args) {
      InputStream inputStream = System.in;
      OutputStream outputStream = System.out;
      InputReader in = new InputReader(inputStream);
      OutputWriter out = new OutputWriter(outputStream);
      BResistorsInParallel solver = new BResistorsInParallel();
      solver.solve(1, in, out);
      out.close();
   }

   static class BResistorsInParallel {
      public void solve(int testNumber, InputReader in, OutputWriter out) {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      }

   }

   static class InputReader {
      private InputStream stream;
      private byte[] buf = new byte[1024];
      private int curChar;
      private int numChars;
      private InputReader.SpaceCharFilter filter;

      public InputReader(InputStream stream) {
         this.stream = stream;
      }

      public int read() {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      }

      public int nextInt() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      }

      public boolean isSpaceChar(int c) {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      }

      public static boolean isWhitespace(int c) {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      }

      public double nextDouble() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      }

      public interface SpaceCharFilter {
         public boolean isSpaceChar(int ch);

      }

   }

   static class OutputWriter {
      private final PrintWriter writer;

      public OutputWriter(OutputStream outputStream) {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      }

      public OutputWriter(Writer writer) {
         this.writer = new PrintWriter(writer);
      }

      public void print(Object... objects) {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      }

      public void println(Object... objects) {
         print(objects);
         writer.println();
      }

      public void close() {
         writer.close();
      }

   }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author AyushKM
 */
public class Main {
   public static void main(String[] args) {
      InputStream inputStream = System.in;
      OutputStream outputStream = System.out;
      InputReader in = new InputReader(inputStream);
      OutputWriter out = new OutputWriter(outputStream);
      BResistorsInParallel solver = new BResistorsInParallel();
      solver.solve(1, in, out);
      out.close();
   }

   static class BResistorsInParallel {
      public void solve(int testNumber, InputReader in, OutputWriter out) {
         int n = in.nextInt();
         double a = 0;
         for (int x = 0; x < n; x++) {
            a += (1.0 / in.nextDouble());
         }
         out.println(1 / a);
      }

   }

   static class InputReader {
      private InputStream stream;
      private byte[] buf = new byte[1024];
      private int curChar;
      private int numChars;
      private InputReader.SpaceCharFilter filter;

      public InputReader(InputStream stream) {
         this.stream = stream;
      }

      public int read() {
         if (numChars == -1) {
            throw new InputMismatchException();
         }
         if (curChar >= numChars) {
            curChar = 0;
            try {
               numChars = stream.read(buf);
            } catch (IOException e) {
               throw new InputMismatchException();
            }
            if (numChars <= 0) {
               return -1;
            }
         }
         return buf[curChar++];
      }

      public int nextInt() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         int res = 0;
         do {
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         } while (!isSpaceChar(c));
         return res * sgn;
      }

      public boolean isSpaceChar(int c) {
         if (filter != null) {
            return filter.isSpaceChar(c);
         }
         return isWhitespace(c);
      }

      public static boolean isWhitespace(int c) {
         return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
      }

      public double nextDouble() {
         int c = read();
         while (isSpaceChar(c)) {
            c = read();
         }
         int sgn = 1;
         if (c == '-') {
            sgn = -1;
            c = read();
         }
         double res = 0;
         while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E') {
               return res * Math.pow(10, nextInt());
            }
            if (c < '0' || c > '9') {
               throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
         }
         if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
               if (c == 'e' || c == 'E') {
                  return res * Math.pow(10, nextInt());
               }
               if (c < '0' || c > '9') {
                  throw new InputMismatchException();
               }
               m /= 10;
               res += (c - '0') * m;
               c = read();
            }
         }
         return res * sgn;
      }

      public interface SpaceCharFilter {
         public boolean isSpaceChar(int ch);

      }

   }

   static class OutputWriter {
      private final PrintWriter writer;

      public OutputWriter(OutputStream outputStream) {
         writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
      }

      public OutputWriter(Writer writer) {
         this.writer = new PrintWriter(writer);
      }

      public void print(Object... objects) {
         for (int i = 0; i < objects.length; i++) {
            if (i != 0) {
               writer.print(' ');
            }
            writer.print(objects[i]);
         }
      }

      public void println(Object... objects) {
         print(objects);
         writer.println();
      }

      public void close() {
         writer.close();
      }

   }
}

