import java.util.Scanner;

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

        System.out.println(Math.pow(sum, -1));
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner std = new Scanner(System.in);
        int n = std.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum += 1 / (double) std.nextInt();
        }

        System.out.println(Math.pow(sum, -1));
    }
} Main public static void main(String[] args) {
        Scanner std = new Scanner(System.in);
        int n = std.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum += 1 / (double) std.nextInt();
        }

        System.out.println(Math.pow(sum, -1));
    } main {
        Scanner std = new Scanner(System.in);
        int n = std.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum += 1 / (double) std.nextInt();
        }

        System.out.println(Math.pow(sum, -1));
    } Scanner std = new Scanner(System.in); std new Scanner(System.in) int n = std.nextInt(); n std.nextInt() std.nextInt std double sum = 0; sum 0 for (int i = 0; i < n; i++) {
            sum += 1 / (double) std.nextInt();
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            sum += 1 / (double) std.nextInt();
        } {
            sum += 1 / (double) std.nextInt();
        } sum += 1 / (double) std.nextInt() sum 1 / (double) std.nextInt() 1 (double) std.nextInt() std.nextInt std System.out.println(Math.pow(sum, -1)) System.out.println System.out System System.out Math.pow(sum, -1) Math.pow Math sum -1 1 String[] args args public class Main {
    public static void main(String[] args) {
        Scanner std = new Scanner(System.in);
        int n = std.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum += 1 / (double) std.nextInt();
        }

        System.out.println(Math.pow(sum, -1));
    }
} public class Main {
    public static void main(String[] args) {
        Scanner std = new Scanner(System.in);
        int n = std.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum += 1 / (double) std.nextInt();
        }

        System.out.println(Math.pow(sum, -1));
    }
} Main
import java.util.Scanner;

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

        System.out.println(Math.pow(sum, -1));
    }
}