#include <bits/stdc++.h>
using namespace std;

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	
	int n; cin >> n;
	vector<long long> dp(n);
	dp[0] = 1; dp[1] = 2;
	for(int i = 2; i < n; i++) dp[i] = dp[i - 1] + dp[i - 2];
	cout << dp[n - 1] << endl;
	return 0;	
}