import java.util.Scanner;

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();
        }
        sc.close();

        double bunbo = 0;
        for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        }
        double ans = 1 / bunbo;
        System.out.println(ans);
    }

} import java.util.Scanner; Scanner 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();
        }
        sc.close();

        double bunbo = 0;
        for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        }
        double ans = 1 / bunbo;
        System.out.println(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();
        }
        sc.close();

        double bunbo = 0;
        for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        }
        double ans = 1 / bunbo;
        System.out.println(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();
        }
        sc.close();

        double bunbo = 0;
        for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        }
        double ans = 1 / bunbo;
        System.out.println(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 sc.close() sc.close sc double bunbo = 0; bunbo 0 for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            bunbo += 1.0 / a[i];
        } {
            bunbo += 1.0 / a[i];
        } bunbo += 1.0 / a[i] bunbo 1.0 / a[i] 1.0 a[i] a i double ans = 1 / bunbo; ans 1 / bunbo 1 bunbo System.out.println(ans) System.out.println System.out System System.out 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();
        }
        sc.close();

        double bunbo = 0;
        for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        }
        double ans = 1 / bunbo;
        System.out.println(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();
        }
        sc.close();

        double bunbo = 0;
        for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        }
        double ans = 1 / bunbo;
        System.out.println(ans);
    }

} Main
import java.util.Scanner;

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();
        }
        sc.close();

        double bunbo = 0;
        for (int i = 0; i < n; i++) {
            bunbo += 1.0 / a[i];
        }
        double ans = 1 / bunbo;
        System.out.println(ans);
    }

}
