using System;
using System.Text.RegularExpressions;
class Program
{
    static void Main(string[] args)
    {
        Double sum = 0;
        Console.ReadLine();
        foreach (string x in Regex.Split(Console.ReadLine(), @"\s+"))
        {
            sum += 1/Convert.ToDouble(x);
        };
        Console.WriteLine("{0}",1.0/sum);
    }
}