import java.util.Scanner;

public class Main {

    void solve() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        double tmp = 0;
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        }
        System.out.print(1/tmp);
    }

    // Main
    public static void main(String[] args) {
        new Main().solve();
    }
} import java.util.Scanner; Scanner java.util public class Main {

    void solve() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        double tmp = 0;
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        }
        System.out.print(1/tmp);
    }

    // Main
    public static void main(String[] args) {
        new Main().solve();
    }
} Main void solve() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        double tmp = 0;
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        }
        System.out.print(1/tmp);
    } solve {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        double tmp = 0;
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        }
        System.out.print(1/tmp);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int[] A = new int[N]; A new int[N] N double tmp = 0; tmp 0 for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        } {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        } A[i] = sc.nextInt() A[i] A i sc.nextInt() sc.nextInt sc tmp += (double)1/A[i] tmp (double)1/A[i] (double)1 A[i] A i System.out.print(1/tmp) System.out.print System.out System System.out 1/tmp 1 tmp // Main
    public static void main(String[] args) {
        new Main().solve();
    } main {
        new Main().solve();
    } new Main().solve() new Main().solve new Main() String[] args args public class Main {

    void solve() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        double tmp = 0;
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        }
        System.out.print(1/tmp);
    }

    // Main
    public static void main(String[] args) {
        new Main().solve();
    }
} public class Main {

    void solve() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        double tmp = 0;
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        }
        System.out.print(1/tmp);
    }

    // Main
    public static void main(String[] args) {
        new Main().solve();
    }
} Main
import java.util.Scanner;

public class Main {

    void solve() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        double tmp = 0;
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
            tmp += (double)1/A[i];
        }
        System.out.print(1/tmp);
    }

    // Main
    public static void main(String[] args) {
        new Main().solve();
    }
}