
import java.util.*;
import java.lang.*;

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

        double sumInverseA = 0;
        for(int i = 0; i < N; i++){
            sumInverseA += inverseA[i];
        }


        System.out.println(1 / sumInverseA);




    }
}


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

        double sumInverseA = 0;
        for(int i = 0; i < N; i++){
            sumInverseA += inverseA[i];
        }


        System.out.println(1 / sumInverseA);




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

        double sumInverseA = 0;
        for(int i = 0; i < N; i++){
            sumInverseA += inverseA[i];
        }


        System.out.println(1 / sumInverseA);




    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int A[] = new int[N];
        double inverseA[] = new double[N];
        for(int i = 0; i < N; i++){
            A[i] = sc.nextInt();
        }
        for(int i = 0; i < N; i++){
            inverseA[i] = 1 / (double)A[i];
        }

        double sumInverseA = 0;
        for(int i = 0; i < N; i++){
            sumInverseA += inverseA[i];
        }


        System.out.println(1 / sumInverseA);




    } 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 double inverseA[] = new double[N]; inverseA new double[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 for(int i = 0; i < N; i++){
            inverseA[i] = 1 / (double)A[i];
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            inverseA[i] = 1 / (double)A[i];
        } {
            inverseA[i] = 1 / (double)A[i];
        } inverseA[i] = 1 / (double)A[i] inverseA[i] inverseA i 1 / (double)A[i] 1 (double)A[i] A i double sumInverseA = 0; sumInverseA 0 for(int i = 0; i < N; i++){
            sumInverseA += inverseA[i];
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            sumInverseA += inverseA[i];
        } {
            sumInverseA += inverseA[i];
        } sumInverseA += inverseA[i] sumInverseA inverseA[i] inverseA i System.out.println(1 / sumInverseA) System.out.println System.out System System.out 1 / sumInverseA 1 sumInverseA String[] args args

import java.util.*;
import java.lang.*;

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

        double sumInverseA = 0;
        for(int i = 0; i < N; i++){
            sumInverseA += inverseA[i];
        }


        System.out.println(1 / sumInverseA);




    }
}



