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

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();
        }
        double gyaku = 0;
        for(int i = 0;i<n;i++){
            gyaku += 1 / (float)(a[i]);
        }
        gyaku = 1 / gyaku;
        System.out.println(gyaku);
        sc.close();
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz