#include<iostream>
#include<algorithm>
#include<vector>
#include<queue>
#include<functional>
#include<set>
#include<string>
using namespace std;
const int surplus = 100000;
int n;
int dp[1000001][2];
int main(){
	cin >> n;
	dp[1][0] = 1;
	for (int i = 2; i <= n; i++){
		dp[i][0] = dp[i - 1][0] + dp[i - 1][1];
		dp[i][1] = dp[i - 1][0];
	}
	cout << dp[n][0] + dp[n][1] << endl;
	return 0;
}