import java.util.*;

public class Main {
public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int n=sc.nextInt();
		int a[]=new int[n];
		for(int s=0;s<n;s++){
			a[s]=sc.nextInt();
		}
		double total=0;

		for(int s=0;s<n;s++){
			total+=(double)1/a[s];
		}

		
		System.out.println((double)1/total);

	}
}

/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