using System;
class Program
{
  static void Main()
  {
    int N = int.Parse(Console.ReadLine());
    string[] A = Console.ReadLine().Split(' ');
    double a = 0;
    for (int i = 0; i < N; i++)
    {
      a += 1 / double.Parse(A[i])
    }
    Console.WriteLine(1 / a);
  }
}