#include<bits/stdc++.h>
using namespace std;
int main(){
	int n;
	cin >> n;
	int a[20];
	a[0] = 0;
	a[1] = 2;
	a[2] = 3;
	for(int i=3;i<=n;i++)
		a[i] = a[i-1] + a[i-2];
	cout << a[n] << endl;	
	return 0;
}