import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        int N = in.nextInt();
        float tempRes = 0;
        for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        }
        System.out.println(1 / tempRes);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        int N = in.nextInt();
        float tempRes = 0;
        for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        }
        System.out.println(1 / tempRes);
    }
} Main public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        int N = in.nextInt();
        float tempRes = 0;
        for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        }
        System.out.println(1 / tempRes);
    } main {
        Scanner in = new Scanner(System.in);

        int N = in.nextInt();
        float tempRes = 0;
        for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        }
        System.out.println(1 / tempRes);
    } Scanner in = new Scanner(System.in); in new Scanner(System.in) int N = in.nextInt(); N in.nextInt() in.nextInt in float tempRes = 0; tempRes 0 for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        } {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        } int A = in.nextInt(); A in.nextInt() in.nextInt in tempRes += (float) 1 / A tempRes (float) 1 / A (float) 1 A System.out.println(1 / tempRes) System.out.println System.out System System.out 1 / tempRes 1 tempRes String[] args args public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        int N = in.nextInt();
        float tempRes = 0;
        for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        }
        System.out.println(1 / tempRes);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        int N = in.nextInt();
        float tempRes = 0;
        for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        }
        System.out.println(1 / tempRes);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        int N = in.nextInt();
        float tempRes = 0;
        for (int i = 0; i < N; i++) {
            int A = in.nextInt();
            tempRes += (float) 1 / A;
        }
        System.out.println(1 / tempRes);
    }
}
