import java.util.Scanner;
public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		double[] array = new double[seisu];
		double sum = 0;
		double gyaku = 0;

		for(int i=0;i<seisu;i++) {
			array[i] = scan.nextInt();
			sum += 1/array[i];
		}
		gyaku = 1/sum;

		System.out.println(gyaku);
		scan.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