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