#include <iostream>
#include <algorithm>
#include <vector>
#include <iomanip>
using namespace std;

typedef long long ll;
typedef pair<int, int> P;
const int INF = (int)1e9 + 1;

ll dp[51];

int main(void) {
	int n;
	cin >> n;
	for (int i = 0; i <= n; i++) {
		dp[i] = 0;
	}
	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;
}