import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        }
        double ans = 0;
        for (int i : a) {
            ans += (double) 1 / i;
        }
        System.out.println((double) 1/ ans);

    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        }
        double ans = 0;
        for (int i : a) {
            ans += (double) 1 / i;
        }
        System.out.println((double) 1/ ans);

    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        }
        double ans = 0;
        for (int i : a) {
            ans += (double) 1 / i;
        }
        System.out.println((double) 1/ ans);

    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        }
        double ans = 0;
        for (int i : a) {
            ans += (double) 1 / i;
        }
        System.out.println((double) 1/ ans);

    } 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 for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        } int i = 0; int i = 0; i 0 i< n i n i++ i++ i {
            a[i] = sc.nextInt();
        } {
            a[i] = sc.nextInt();
        } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc double ans = 0; ans 0 for (int i : a) {
            ans += (double) 1 / i;
        } int i a {
            ans += (double) 1 / i;
        } {
            ans += (double) 1 / i;
        } ans += (double) 1 / i ans (double) 1 / i (double) 1 i System.out.println((double) 1/ ans) System.out.println System.out System System.out (double) 1/ ans (double) 1 ans String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        }
        double ans = 0;
        for (int i : a) {
            ans += (double) 1 / i;
        }
        System.out.println((double) 1/ ans);

    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        }
        double ans = 0;
        for (int i : a) {
            ans += (double) 1 / i;
        }
        System.out.println((double) 1/ ans);

    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i< n; i++) {
            a[i] = sc.nextInt();
        }
        double ans = 0;
        for (int i : a) {
            ans += (double) 1 / i;
        }
        System.out.println((double) 1/ ans);

    }
}
