import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        scanner.nextLine();
        String[] paramsA = scanner.nextLine().split(" ");
        scanner.close();

        double denominator = 0;
        
        for (String a: paramsA) {
            denominator += (double)1 / (double)(Integer.parseInt(a));
        }

        System.out.println(String.valueOf(((double)1 / denominator)));
    }
}
/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