import java.util.*;
class Main
{
  public static void main(String []args)
  {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int z=0;
    double d=0.0;
    for(int i=0;i<a;i++)
    {
      z=sc.nextInt();
      d=d+1.0/z;
    }
    d=1/d;
    System.out.println(d);
    }
}
/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