import java.util.*;

class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        double[] A = new double[a];
        double sum = 0.0;
        double seki = 1.0;
        for(int i = 0;i < a;i++){
            A[i] = 1.0/sc.nextDouble();
            sum += A[i];
            
        } 

        double ans = 1.0/sum;
        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