#include <iostream>
#include <cstdio>

using namespace std;

const int N = 2e5 + 100;


int dp[N][2];
int n;

int main(){
	cin >> n;
	dp[1][0] = 1;
	dp[1][1] = 1;
	for(int i = 2; i <= n; i ++){
		dp[i][1] += dp[i - 1][0];
		dp[i][0] += dp[i - 1][1] + dp[i - 1][0];
	}
	cout << dp[n][1] + dp[n][0] << endl;
}