#include<iostream>
#include<algorithm>
using namespace std;
using ll = long long;

int main() {
	ll n;
	cin >> n;
	ll a[60] = {};
	a[1] = 1;
	a[2] = 2;
	for (int i = 3;i <= n;i++) {
		a[i] = a[i - 2] + a[i - 1];
	}
	cout << a[n] << endl;
}