import java.util.Scanner;

public class Main{
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        double sumaFraccion = 0;
        int n = in.nextInt();
        for (int i = 0; i < n; i++) {
            double Ai= in.nextDouble();
            sumaFraccion+= 1.0/Ai;
            
        }
        double resutlado = 1/sumaFraccion;
        System.out.println(resutlado);
    }
}
/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