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