import java.util.*;

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

    int n = sc.nextInt();
    double[] array = new double[n];
    double a, sum = 0 ;

    for(int i=0; i<n; i++){
      a = sc.nextInt();
      a = 1 /(double)a;
      sum += a;
    }
    sum = 1 /sum ;
    System.out.println(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