import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws Exception {
        try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        }
    }

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

    public static void main(String[] args) throws Exception {
        try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        }
    }

} Main public static void main(String[] args) throws Exception {
        try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        }
    } main {
        try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        }
    } try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        } {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        } Scanner sc = new Scanner(System.in) Scanner sc = new Scanner(System.in) new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double d = 0; d 0 for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
                int a = sc.nextInt();
                d += 1.0 / a;
            } {
                int a = sc.nextInt();
                d += 1.0 / a;
            } int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc d += 1.0 / a d 1.0 / a 1.0 a System.out.println(1.0 / d) System.out.println System.out System System.out 1.0 / d 1.0 d String[] args args public class Main {

    public static void main(String[] args) throws Exception {
        try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        }
    }

} public class Main {

    public static void main(String[] args) throws Exception {
        try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        }
    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws Exception {
        try (Scanner sc = new Scanner(System.in)) {
            int N = sc.nextInt();

            double d = 0;
            for (int i = 0; i < N; i++) {
                int a = sc.nextInt();
                d += 1.0 / a;
            }

            System.out.println(1.0 / d);
        }
    }

}
