using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace contest
{
    class Program
    {
        static void Main(string[] args)
        {
            long N = long.Parse(Console.ReadLine());

            long[] array = new long[50];

            array[0] = 1;
            array[1] = 2;

            for (int i = 2; i < N; i++)
            {
                array[i] = array[i - 2] + array[i - 1]; 
            }

            Console.WriteLine(array[N - 1]);
        }
    }
}