using System;
using System.Collections.Generic;
using System.Linq;

namespace yukicoder
{
    public class Program
    {
        public static void Main()
        {
            var n = double.Parse(Console.ReadLine())+1;
            var a = Math.Sqrt(5);
            var fn = 1 / a * (Math.Pow((1 + a) / 2, n) - Math.Pow((1 - a) / 2, n));
            Console.WriteLine(fn);
        }
    }
}