import java.util.Scanner;

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


/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