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

namespace yukicoder
{
    public class Program
    {
        public static void Main()
        {
            var n = int.Parse(Console.ReadLine());
            var p = new long[n];
            p[0] = 1;
            p[1] = 2;
            for(var i = 2; i < n; i++)
            {
                p[i] = p[i - 1] + p[i - 2];
            }
            Console.WriteLine(p[n - 1]);
        }
    }
}