#include<iostream>

unsigned long long field[51];

unsigned long long func(unsigned long long n){
	if (n == 1)return field[n] = 1;
	if (n == 2)return field[n] = 2;
	if (field[n] != 0)return field[n];
	return field[n] = func(n - 1) + func(n - 2);
}

int main(){
	unsigned long long n;
	std::cin >> n;
	
	//for (int i = 0; i < 50; ++i)std::cout << field[i]<<" ";
	std::cout << func(n) << std::endl;
	return 0;
}