#include<bits/stdc++.h>
#define LL long long
using namespace std;
const int N=1e7+5;
int a[N]{0,1,2,3};
int main(){
	int n;
	cin>>n;
	for(int i=4;i<=n;i++){
		a[i]=a[i-1]+a[i-2];
	}
	cout<<a[n];
	return 0;
}