#include<bits/stdc++.h>
using namespace std;
int n,ans;
void ss(int x){
	for(int i=1;i<=x/2;i++){
		ans++;
		ss(i);
	}
}
int main(){
	int n;
	cin>>n;
	ss(n);
	cout<<ans;
	return 0;
}

/**************************************************************
	Problem: 1307
	User: zzz
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/