import java.util.*;
public class Main {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    float[] A;
    A = new float[N+1];
    float tmp=0;
    for (int i= 1; i <= N; i++){
      A[i] = scan.nextFloat();
      tmp= tmp + 1/A[i];
    }
      System.out.println(1/tmp);
  
  }
}
/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