import java.util.*;

class Main {
    public static void main(String[] args) throws java.lang.Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        double sum = 0d;
        for (int i = 0; i < n; i++) {
            sum += 1 / sc.nextDouble();
        }
        System.out.println(1 / sum);
        sc.close();
    }
} import java.util.*; util.* java class Main {
    public static void main(String[] args) throws java.lang.Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        double sum = 0d;
        for (int i = 0; i < n; i++) {
            sum += 1 / sc.nextDouble();
        }
        System.out.println(1 / sum);
        sc.close();
    }
} Main public static void main(String[] args) throws java.lang.Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        double sum = 0d;
        for (int i = 0; i < n; i++) {
            sum += 1 / sc.nextDouble();
        }
        System.out.println(1 / sum);
        sc.close();
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        double sum = 0d;
        for (int i = 0; i < n; i++) {
            sum += 1 / sc.nextDouble();
        }
        System.out.println(1 / sum);
        sc.close();
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double sum = 0d; sum 0d 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 System.out.println(1 / sum) System.out.println System.out System System.out 1 / sum 1 sum sc.close() sc.close sc String[] args args
import java.util.*;

class Main {
    public static void main(String[] args) throws java.lang.Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        double sum = 0d;
        for (int i = 0; i < n; i++) {
            sum += 1 / sc.nextDouble();
        }
        System.out.println(1 / sum);
        sc.close();
    }
}
