import java.util.*;
import java.util.Collections;

public class Main {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double total = 0;
    for (int i = 0; i < N; i++) {
      total += 1 / sc.nextDouble();
    }
    System.out.println(1 / total);
  }

  public static void primeFactorization(int num) {
    int Z = 0;
    for (int i = 2; num > i; i++) {
      while (num % i == 0) {
        System.out.println(i);
        num /= i;
        Z = num;
      }
    }
    System.out.println(Z);
  }

  public static boolean isPrime(int num) {
    if (num < 2)
      return false;
    else if (num == 2)
      return true;
    else if (num % 2 == 0)
      return false;

    for (int i = 3; i < num; i += 2) {
      if (num % i == 0) {
        return false;
      }
    }
    return true;
  }
}

/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