import java.util.Scanner;


// 138_a
public class Main {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        int n = scan.nextInt();

        double tmp = 0.0;
        for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        }

        System.out.println(1.0 / tmp);

    }

} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        int n = scan.nextInt();

        double tmp = 0.0;
        for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        }

        System.out.println(1.0 / tmp);

    }

} Main public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        int n = scan.nextInt();

        double tmp = 0.0;
        for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        }

        System.out.println(1.0 / tmp);

    } main {

        Scanner scan = new Scanner(System.in);

        int n = scan.nextInt();

        double tmp = 0.0;
        for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        }

        System.out.println(1.0 / tmp);

    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int n = scan.nextInt(); n scan.nextInt() scan.nextInt scan double tmp = 0.0; tmp 0.0 for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            tmp += 1.0 / scan.nextDouble();
        } {
            tmp += 1.0 / scan.nextDouble();
        } tmp += 1.0 / scan.nextDouble() tmp 1.0 / scan.nextDouble() 1.0 scan.nextDouble() scan.nextDouble scan System.out.println(1.0 / tmp) System.out.println System.out System System.out 1.0 / tmp 1.0 tmp String[] args args public class Main {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        int n = scan.nextInt();

        double tmp = 0.0;
        for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        }

        System.out.println(1.0 / tmp);

    }

} public class Main {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        int n = scan.nextInt();

        double tmp = 0.0;
        for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        }

        System.out.println(1.0 / tmp);

    }

} Main
import java.util.Scanner;


// 138_a
public class Main {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        int n = scan.nextInt();

        double tmp = 0.0;
        for (int i = 0; i < n; i++) {
            tmp += 1.0 / scan.nextDouble();
        }

        System.out.println(1.0 / tmp);

    }

}
