import java.util.*;
public class Main {
 public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int N=sc.nextInt();

	double bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 }
}

/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