using System;

public class Hello
{
    static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        Console.WriteLine(getAns(n));
    }
    static long getAns (int n)
    {
        var a = new long[n + 1];
        a[1] = 1;  a[2] = 2;
        for (int i = 3; i <= n; i++)
            a[i] = a[i - 1] + a[i - 2];
        return a[n];
    }
}