/*
* 786_sample.cpp
* 
* 
*/

#include <iostream>
#define MAX 50

int main () {

	auto n = 0;

	std::cin >> n;
	int x[MAX] = {};
	x[0] = 1;
	x[1] = 2;

	for (auto i = 2; i < n; i++) {
		x[i] = x[i-2] + x[i-1];
	}
	std::cout << x[n-1] << std::endl;
	
	return 0;
	
}