import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double inv = 0;
        for (int i = 0, lim = sc.nextInt(); i < lim; i++) {
            inv += (1.0 / sc.nextInt());
        }
        System.out.println(1.0 / inv);
    }
} import java.util.Scanner; Scanner java.util class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double inv = 0;
        for (int i = 0, lim = sc.nextInt(); i < lim; i++) {
            inv += (1.0 / sc.nextInt());
        }
        System.out.println(1.0 / inv);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double inv = 0;
        for (int i = 0, lim = sc.nextInt(); i < lim; i++) {
            inv += (1.0 / sc.nextInt());
        }
        System.out.println(1.0 / inv);
    } main {
        Scanner sc = new Scanner(System.in);
        double inv = 0;
        for (int i = 0, lim = sc.nextInt(); i < lim; i++) {
            inv += (1.0 / sc.nextInt());
        }
        System.out.println(1.0 / inv);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double inv = 0; inv 0 for (int i = 0, lim = sc.nextInt(); i < lim; i++) {
            inv += (1.0 / sc.nextInt());
        } int i = 0, lim = sc.nextInt(); int i = 0 i 0 lim = sc.nextInt(); lim sc.nextInt() sc.nextInt sc i < lim i lim i++ i++ i {
            inv += (1.0 / sc.nextInt());
        } {
            inv += (1.0 / sc.nextInt());
        } inv += (1.0 / sc.nextInt()) inv (1.0 / sc.nextInt()) 1.0 sc.nextInt() sc.nextInt sc System.out.println(1.0 / inv) System.out.println System.out System System.out 1.0 / inv 1.0 inv String[] args args
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double inv = 0;
        for (int i = 0, lim = sc.nextInt(); i < lim; i++) {
            inv += (1.0 / sc.nextInt());
        }
        System.out.println(1.0 / inv);
    }
}