import java.util.Scanner;

public class Main {

	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	}
}
/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