import java.util.*;
public class Main {

	public static void main(String[] ble){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double inv = 0;
		for(int i = 0; i < n; i++){
			double a = sc.nextDouble();
			inv += 1/a;
		}
		System.out.println(1/inv);
	}
}

/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