#include<iostream>
#include<cmath>
#include<algorithm>
#include<string>
#include<map>
using namespace std;
int sum(int n) {
	if (n == 1) {
		return 2;
	}
	if (n == 2) {
		return 3;
	}
	return sum(n - 1) + sum(n - 2);
}
int main() {
	int n;
	cin >> n;
	cout << sum(n);
	return 0;
}