#include <iostream>
using namespace std;
int main(){
	int n;
	unsigned long long a = 1, b = 2, c;
	cin >> n;
	if (n == 1){
		c = 1;
	}
	else if (n == 2){
		c = 2;
	}
	for (int i = 3; i <= n; i ++){
		c = a + b;
		a = b; b = c;
	}
	cout << c;
}