import java.util.Scanner;

public class Main {
    static Scanner stdIn = new Scanner(System.in);
    public static void main(String[] args){
        int n = stdIn.nextInt();
        long[] a = new long[n];
        for(int i=0; i<n; i++){
            a[i] = stdIn.nextLong();
        }

        double inverseSum = 0.0;
        for(int i=0; i<n; i++){
            inverseSum += ((double)1 / a[i]);
        }

        System.out.println((double)1 / inverseSum);
       }
}
/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