import java.util.Scanner;
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 j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    }
}

/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