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();
            a[i] = 1 / a[i];
            if(i != 0){
                a[0] += a[i];
            }
        }
        System.out.println(1 / a[0]);
    }
}
/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