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];
        double ans = 0.0;
        for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        }
        System.out.println(1 / 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];
        double ans = 0.0;
        for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        }
        System.out.println(1 / ans);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        double ans = 0.0;
        for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        }
        System.out.println(1 / ans);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        double ans = 0.0;
        for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        }
        System.out.println(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 double ans = 0.0; ans 0.0 for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            ans += 1.0 / (double)sc.nextDouble();
        } {
            ans += 1.0 / (double)sc.nextDouble();
        } ans += 1.0 / (double)sc.nextDouble() ans 1.0 / (double)sc.nextDouble() 1.0 (double)sc.nextDouble() sc.nextDouble sc System.out.println(1 / ans) System.out.println System.out System System.out 1 / ans 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];
        double ans = 0.0;
        for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        }
        System.out.println(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];
        double ans = 0.0;
        for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        }
        System.out.println(1 / 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];
        double ans = 0.0;
        for(int i=0; i<n; i++) {
            ans += 1.0 / (double)sc.nextDouble();
        }
        System.out.println(1 / ans);
    }
}