import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int n=scan.nextInt();
        int a;
        double b=0;
        double ans;

        for(int i=0;i<n;i++){
            a=scan.nextInt();
            b+=1.0/a;
//            System.out.println(b);
        }

        ans=1/b;
        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