import java.util.*;

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


        System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A));
    }
    public static double RegistersInParallel(int N, int index, int[] A) {
        if (N <= index) {
            return 0.0;
        }
        return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
        }
        sc.close();


        System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A));
    }
    public static double RegistersInParallel(int N, int index, int[] A) {
        if (N <= index) {
            return 0.0;
        }
        return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
        }
        sc.close();


        System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A));
    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
        }
        sc.close();


        System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A));
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int[] A = new int[N]; A new int[N] N for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            A[i] = sc.nextInt();
        } {
            A[i] = sc.nextInt();
        } A[i] = sc.nextInt() A[i] A i sc.nextInt() sc.nextInt sc sc.close() sc.close sc System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A)) System.out.printf System.out System System.out "%.10f" 1.0 / RegistersInParallel(N, 0, A) 1.0 RegistersInParallel(N, 0, A) RegistersInParallel N 0 A String[] args args public static double RegistersInParallel(int N, int index, int[] A) {
        if (N <= index) {
            return 0.0;
        }
        return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A);
    } RegistersInParallel {
        if (N <= index) {
            return 0.0;
        }
        return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A);
    } if (N <= index) {
            return 0.0;
        } N <= index N index {
            return 0.0;
        } return 0.0; 0.0 return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A); (1.0 / A[index]) + RegistersInParallel(N, index + 1, A) (1.0 / A[index]) 1.0 A[index] A index RegistersInParallel(N, index + 1, A) RegistersInParallel N index + 1 index 1 A int N N int index index int[] A A public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
        }
        sc.close();


        System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A));
    }
    public static double RegistersInParallel(int N, int index, int[] A) {
        if (N <= index) {
            return 0.0;
        }
        return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N];
        for (int i = 0; i < N; i++) {
            A[i] = sc.nextInt();
        }
        sc.close();


        System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A));
    }
    public static double RegistersInParallel(int N, int index, int[] A) {
        if (N <= index) {
            return 0.0;
        }
        return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A);
    }
} Main
import java.util.*;

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


        System.out.printf("%.10f", 1.0 / RegistersInParallel(N, 0, A));
    }
    public static double RegistersInParallel(int N, int index, int[] A) {
        if (N <= index) {
            return 0.0;
        }
        return (1.0 / A[index]) + RegistersInParallel(N, index + 1, A);
    }
}
