import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class Main { 

	public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int inputScore = Integer.parseInt(sc.nextLine());
    	String[] inputNumber = sc.nextLine().split(" ");
    	double[] allNumber = new double[inputScore];
    	double all = 0;
    	for(int i = 0;i<inputScore;i++) {
    		double tmp = Double.parseDouble(inputNumber[i]);
    		all += 1/tmp;
    	}

    	
    	System.out.println(1/all);
    }
}
/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