
import java.io.*;
import java.util.StringTokenizer;
public class Main{
    public static void main(String[] args) throws IOException {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    }
}
class scanner{
    StringTokenizer st;
    BufferedReader br;
    public scanner(InputStream s){
        br = new BufferedReader(new InputStreamReader(s));
    }
    public scanner (String s) throws FileNotFoundException { br = new BufferedReader(new FileReader(s));}

    public String next() throws IOException {
        while (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(br.readLine());
        return st.nextToken();
    }

    public int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    public long nextLong() throws IOException {
        return  Long.parseLong(next());
    }

    public String nextLine() throws IOException {
        return br.readLine();
    }

    public double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    public boolean ready() throws IOException {
        return br.ready();
    }
} import java.io.*; io.* java import java.util.StringTokenizer; StringTokenizer java.util public class Main{
    public static void main(String[] args) throws IOException {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    }
} Main public static void main(String[] args) throws IOException {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    } main {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    } scanner sc = new scanner(System.in); sc new scanner(System.in) PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double sum = 0; sum 0 for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            sum+= 1/sc.nextDouble();
        } {
            sum+= 1/sc.nextDouble();
        } sum+= 1/sc.nextDouble() sum 1/sc.nextDouble() 1 sc.nextDouble() sc.nextDouble sc out.print(1/sum) out.print out 1/sum 1 sum out.flush() out.flush out String[] args args class scanner{
    StringTokenizer st;
    BufferedReader br;
    public scanner(InputStream s){
        br = new BufferedReader(new InputStreamReader(s));
    }
    public scanner (String s) throws FileNotFoundException { br = new BufferedReader(new FileReader(s));}

    public String next() throws IOException {
        while (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(br.readLine());
        return st.nextToken();
    }

    public int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    public long nextLong() throws IOException {
        return  Long.parseLong(next());
    }

    public String nextLine() throws IOException {
        return br.readLine();
    }

    public double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    public boolean ready() throws IOException {
        return br.ready();
    }
} scanner StringTokenizer st; st BufferedReader br; br public scanner(InputStream s){
        br = new BufferedReader(new InputStreamReader(s));
    } scanner {
        br = new BufferedReader(new InputStreamReader(s));
    } br = new BufferedReader(new InputStreamReader(s)) br new BufferedReader(new InputStreamReader(s)) InputStream s s public scanner (String s) throws FileNotFoundException { br = new BufferedReader(new FileReader(s));} scanner { br = new BufferedReader(new FileReader(s));} br = new BufferedReader(new FileReader(s)) br new BufferedReader(new FileReader(s)) String s s public String next() throws IOException {
        while (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(br.readLine());
        return st.nextToken();
    } next {
        while (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(br.readLine());
        return st.nextToken();
    } while (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(br.readLine()); st == null || !st.hasMoreTokens() st == null st null !st.hasMoreTokens() st.hasMoreTokens() st.hasMoreTokens st st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st public int nextInt() throws IOException {
        return Integer.parseInt(next());
    } nextInt {
        return Integer.parseInt(next());
    } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next public long nextLong() throws IOException {
        return  Long.parseLong(next());
    } nextLong {
        return  Long.parseLong(next());
    } return  Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next public String nextLine() throws IOException {
        return br.readLine();
    } nextLine {
        return br.readLine();
    } return br.readLine(); br.readLine() br.readLine br public double nextDouble() throws IOException {
        return Double.parseDouble(next());
    } nextDouble {
        return Double.parseDouble(next());
    } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next public boolean ready() throws IOException {
        return br.ready();
    } ready {
        return br.ready();
    } return br.ready(); br.ready() br.ready br public class Main{
    public static void main(String[] args) throws IOException {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    }
} public class Main{
    public static void main(String[] args) throws IOException {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    }
} Main

import java.io.*;
import java.util.StringTokenizer;
public class Main{
    public static void main(String[] args) throws IOException {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    }
}
class scanner{
    StringTokenizer st;
    BufferedReader br;
    public scanner(InputStream s){
        br = new BufferedReader(new InputStreamReader(s));
    }
    public scanner (String s) throws FileNotFoundException { br = new BufferedReader(new FileReader(s));}

    public String next() throws IOException {
        while (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(br.readLine());
        return st.nextToken();
    }

    public int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    public long nextLong() throws IOException {
        return  Long.parseLong(next());
    }

    public String nextLine() throws IOException {
        return br.readLine();
    }

    public double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    public boolean ready() throws IOException {
        return br.ready();
    }
}