using System;

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