#include<iostream>
#include<vector>
using namespace std;
int main() {
	int N;
	cin >> N;
	vector<int> a(N + 1);
	int tmp=0;
	if (N <= 1) {
		return 1;
	}
	else {
		a[0] = 1;
		a[1] = 2;
		for (int i = 2; i <N; i++) {
			a[i] = a[i - 1] + a[i - 2];
		}
	}
	for (int i = 0; i < N; i++) {
		tmp += a[i];
	}
	cout << a[N-1];
	return 0;
}