import java.util.*;

public class Main {
	public static void main(String[] args) {
		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 sum = 0;
		double ans = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		ans = 1/sum;
		System.out.println(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