#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,i=1,a;
	cin>>n;
	while(i<=n){
		a=a+i;
		i=i+2;
	}
	a--;
	cout<<a;
}

/**************************************************************
	Problem: 1003
	User: linmiaoling
	Language: C++
	Result: Wrong Answer
****************************************************************/