#include <iostream>
double kaijou(int a) {
	double r = 1;
	while (a > 1) {
		r *= a;
		--a;
	}
	return r;
}
int main() {
	int n;
	std::cin >> n;
	int m2;
	m2 = n / 2;
	int t1;
	double count = 0;
	for (int t2 = 0; t2 <= m2; ++t2) {
		t1 = n - 2 * t2;
		count += kaijou(t1 + t2) / (kaijou(t1) * kaijou(t2));
	}
	std::cout << (long long)count << std::endl;
}