#include <iostream>

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(0);

	int n;
	std::cin >> n;
	long long int step1 = 1, step2 = 2;
	long long int step;

	for (int i = 2; i < n; ++i) {
		step = step1 + step2;
		step1 = step2;
		step2 = step;
	}

	std::cout << ((n==2) ? step2 : step) << "\n";

	return 0;
}