import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        ABC138_b solver = new ABC138_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class ABC138_b {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        }

    }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        ABC138_b solver = new ABC138_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class ABC138_b {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        ABC138_b solver = new ABC138_b();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        ABC138_b solver = new ABC138_b();
        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 Scanner in = new Scanner(inputStream); in new Scanner(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) ABC138_b solver = new ABC138_b(); solver new ABC138_b() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class ABC138_b {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        }

    } ABC138_b public void solve(int testNumber, Scanner in, PrintWriter out) {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        } solve {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        } int N = in.nextInt(); N in.nextInt() in.nextInt in double Adash = 0; Adash 0 int i = 0; i 0 for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            } i = 0; i = 0 i 0 i < N i N i++ i++ i {
                double work = 1 / in.nextDouble();
                Adash += work;
            } {
                double work = 1 / in.nextDouble();
                Adash += work;
            } double work = 1 / in.nextDouble(); work 1 / in.nextDouble() 1 in.nextDouble() in.nextDouble in Adash += work Adash work out.println(1 / Adash) out.println out 1 / Adash 1 Adash int testNumber testNumber Scanner in in PrintWriter out out public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        ABC138_b solver = new ABC138_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class ABC138_b {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        ABC138_b solver = new ABC138_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class ABC138_b {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        ABC138_b solver = new ABC138_b();
        solver.solve(1, in, out);
        out.close();
    }

    static class ABC138_b {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int N = in.nextInt();
            double Adash = 0;
            int i = 0;
            for (i = 0; i < N; i++) {
                double work = 1 / in.nextDouble();
                Adash += work;
            }
            out.println(1 / Adash);

//        int[] A = new int[N];

        }

    }
}

