import java.util.*;

class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        double[] A = new double[a];
        double sum = 0.0;
        double seki = 1.0;
        for(int i = 0;i < a;i++){
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } 

        double ans = 1.0/sum;
        System.out.println(ans);

    }
} import java.util.*; util.* java class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        double[] A = new double[a];
        double sum = 0.0;
        double seki = 1.0;
        for(int i = 0;i < a;i++){
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } 

        double ans = 1.0/sum;
        System.out.println(ans);

    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        double[] A = new double[a];
        double sum = 0.0;
        double seki = 1.0;
        for(int i = 0;i < a;i++){
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } 

        double ans = 1.0/sum;
        System.out.println(ans);

    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        double[] A = new double[a];
        double sum = 0.0;
        double seki = 1.0;
        for(int i = 0;i < a;i++){
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } 

        double ans = 1.0/sum;
        System.out.println(ans);

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc double[] A = new double[a]; A new double[a] a double sum = 0.0; sum 0.0 double seki = 1.0; seki 1.0 for(int i = 0;i < a;i++){
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } int i = 0; int i = 0; i 0 i < a i a i++ i++ i {
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } {
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } A[i] = 1.0/sc.nextDouble() A[i] A i 1.0/sc.nextDouble() 1.0 sc.nextDouble() sc.nextDouble sc sum += A[i] sum A[i] A i double ans = 1.0/sum; ans 1.0/sum 1.0 sum System.out.println(ans) System.out.println System.out System System.out ans String[] args args
import java.util.*;

class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        double[] A = new double[a];
        double sum = 0.0;
        double seki = 1.0;
        for(int i = 0;i < a;i++){
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } 

        double ans = 1.0/sum;
        System.out.println(ans);

    }
}