using System;
using System.Linq;

namespace killme
{
    public static class Program
    {
        public static void Main()
        {
            Console.ReadLine();

            var ret = 1.0 / Console.ReadLine()
                .Split(' ')
                .Select(x => double.Parse(x))
                .Select(x => 1.0 / x)
                .Sum();

            Console.WriteLine(ret);
        }
    }
}
