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

/**************************************************************
	Problem: 1307
	User: fzy001
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/