import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  }
}

/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