import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		
		int N = scan.nextInt();
		int[] number = new int[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			number[i] = scan.nextInt();
			sum += (double)1 / number[i];
		}
	
		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