#include<iostream>
using namespace std;
int f(int h){
	if(h==0)return 0;
	if(h==2)return 2;
	if(h==1)return 1;
	return f(h-1)+f(h-2);
}
int main(){
	int n;
	cin>>n;
	cout<<f(n);
	return 0;
}