import java.util.*;
public class Main{
	public static void main(String[] arugs){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double[] A = new double[N];
		double ans = 0;
		double sum = 0;
		double result = 0;
		
		for(int i=0; i<N; i++){
			A[i] = sc.nextDouble();
		}
		
		for(int j=0; j<N; j++){
			ans = 1/A[j];
			sum += ans;
			ans = 0;
		}
		
		result = 1/sum;
		
		System.out.println(result);
	}
}

/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