import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {

			int n = sc.nextInt();
			
			double[] a = new double[n];
			for(int i = 0 ; i < n ; i++ ) {
				a[i] = sc.nextDouble();
			}
			
			double base = 0;
			for(int i = 0 ; i < n ; i++ ) {
				base += 1f/a[i];
			}
			
			double answer = 1f / base;
			System.out.println(answer);
		}
	}
}

/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