#include<bits/stdc++.h>
using namespace std;
int Fi(unsigned long long n){
	if(n==2)return 2;
	if(n==1)return 1;
	else return Fi(n-1)+Fi(n-2);
}
int main(){
	unsigned long long a;
	cin>>a;
	unsigned long long m=Fi(a);
	cout<<m;
}