import java.util.Scanner;
public class Main{
  public static void main(String[]args){

    Scanner scan = new Scanner(System.in);

    double ans = 0;
    int c = scan.nextInt();
    for(int i=0;i<c;i++){
      double t = (double) scan.nextInt();
      ans += 1.0/t;
    }
    
    System.out.println(1.0/ans);
  }
}
/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