using System;
class Program
{
	static void Main(string[] args)
	{
      double a = double.Parse(Console.ReadLine());
      double b = 0.0f;
      for(int i = 0; i < a;i++){
        b += 1 / double.Parse(Console.ReadLine());
      }
      Console.Write((1/b).ToString());
	}
}