import java.util.*;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] Z = new int[N];
        double A = 0;
        int n = 0;
        int m = 0;
        for(n = 0; n < N; n++) {
            Z[n] = sc.nextInt();
        }

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

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

        while(m < N) {
            double i = (double)1/(double)Z[m];
            A = A + i;
            m++;
        }
        System.out.println((double)(1/A));
    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] Z = new int[N];
        double A = 0;
        int n = 0;
        int m = 0;
        for(n = 0; n < N; n++) {
            Z[n] = sc.nextInt();
        }

        while(m < N) {
            double i = (double)1/(double)Z[m];
            A = A + i;
            m++;
        }
        System.out.println((double)(1/A));
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int[] Z = new int[N]; Z new int[N] N double A = 0; A 0 int n = 0; n 0 int m = 0; m 0 for(n = 0; n < N; n++) {
            Z[n] = sc.nextInt();
        } n = 0; n = 0 n 0 n < N n N n++ n++ n {
            Z[n] = sc.nextInt();
        } {
            Z[n] = sc.nextInt();
        } Z[n] = sc.nextInt() Z[n] Z n sc.nextInt() sc.nextInt sc while(m < N) {
            double i = (double)1/(double)Z[m];
            A = A + i;
            m++;
        } m < N m N {
            double i = (double)1/(double)Z[m];
            A = A + i;
            m++;
        } double i = (double)1/(double)Z[m]; i (double)1/(double)Z[m] (double)1 (double)Z[m] Z m A = A + i A A + i A i m++ m System.out.println((double)(1/A)) System.out.println System.out System System.out (double)(1/A) 1 A String[] args args
import java.util.*;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] Z = new int[N];
        double A = 0;
        int n = 0;
        int m = 0;
        for(n = 0; n < N; n++) {
            Z[n] = sc.nextInt();
        }

        while(m < N) {
            double i = (double)1/(double)Z[m];
            A = A + i;
            m++;
        }
        System.out.println((double)(1/A));
    }
}