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 a = new long[n];
            a[0] = 1;
            a[1] = 2;
            if (n > 2)
            {
                for(var i = 2; i < n; i++)
                {
                    a[i] = a[i - 1] + a[i - 2];
                }
            }
            Console.WriteLine(a[n - 1]);
        }
    }
}