#include <iostream>
using namespace std;
int main() {
	long long dp[51] = { 0 };
	int N; cin >> N;
	dp[1] = 1; dp[2] = 2;
	for (int i = 3; i <= N;i++) {
		dp[i] = dp[i - 1] + dp[i - 2];
	}
	cout << dp[N] << endl;
	return 0;
}