import java.util.Scanner;

class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		int a[] = new int[n];
		
		double ans = 0;
		for(int i=0;i<n;i++) {
			a[i] = s.nextInt();
			ans+=(1.0/a[i]);
		}
		
		System.out.format("%.10f",1.0/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