#include<bits/stdc++.h>
using namespace std;
int main()
{
	long long a[55];
	int n;
	cin >> n;
	a[1] = 1;
	a[2] = 2;
	for (int i = 3; i <= n ; i ++ )
    	{
		a[i] = a[i - 1] + a[i - 2] ;
	}
	cout << a[n] << endl;
	return 0;
}