import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		
		double ans=0;
		double rev_sum=0;
		double a =0;
		
		for(int i=0;i<n;i++) {
			a = sc.nextInt();
			rev_sum += 1/a;
		}
		ans = 1/rev_sum;
		
		System.out.println(ans);
		
		
		
		
		
	}
}

/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