import java.util.Scanner;

public class Main {


    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        main2(sc);
    }

    public static void main2(Scanner sc) {

        int N = sc.nextInt();
        double sum = 0;

        for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        }

        System.out.println(1/sum);
    }
} import java.util.Scanner; Scanner java.util public class Main {


    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        main2(sc);
    }

    public static void main2(Scanner sc) {

        int N = sc.nextInt();
        double sum = 0;

        for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        }

        System.out.println(1/sum);
    }
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        main2(sc);
    } main {
        Scanner sc = new Scanner(System.in);
        main2(sc);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) main2(sc) main2 sc String[] args args public static void main2(Scanner sc) {

        int N = sc.nextInt();
        double sum = 0;

        for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        }

        System.out.println(1/sum);
    } main2 {

        int N = sc.nextInt();
        double sum = 0;

        for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        }

        System.out.println(1/sum);
    } int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double sum = 0; sum 0 for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
        	double x = sc.nextDouble();
        	sum += 1/x;
        } {
        	double x = sc.nextDouble();
        	sum += 1/x;
        } double x = sc.nextDouble(); x sc.nextDouble() sc.nextDouble sc sum += 1/x sum 1/x 1 x System.out.println(1/sum) System.out.println System.out System System.out 1/sum 1 sum Scanner sc sc public class Main {


    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        main2(sc);
    }

    public static void main2(Scanner sc) {

        int N = sc.nextInt();
        double sum = 0;

        for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        }

        System.out.println(1/sum);
    }
} public class Main {


    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        main2(sc);
    }

    public static void main2(Scanner sc) {

        int N = sc.nextInt();
        double sum = 0;

        for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        }

        System.out.println(1/sum);
    }
} Main
import java.util.Scanner;

public class Main {


    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        main2(sc);
    }

    public static void main2(Scanner sc) {

        int N = sc.nextInt();
        double sum = 0;

        for (int i = 0; i < N; i++) {
        	double x = sc.nextDouble();
        	sum += 1/x;
        }

        System.out.println(1/sum);
    }
}