#include<iostream>
#include<algorithm>
#include<string>
#include<vector>

using namespace std;


vector<long> memo(51,-1);
long dp(int N){
	if(memo[N] == -1){
		if(N <= 2){
			memo[N] = N;
			return N;
		}
		else{
			return dp(N-1) + dp(N-2);
		}
	}
	else
		return memo[N];

}

int main(){
	int N;
	cin >> N;
	long long ans;
	ans = dp(N);

	cout << ans << endl;
	return 0;
}