import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);

      int N = sc.nextInt();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    }
}
/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