#include <bits/stdc++.h>
using namespace std;
int main(){
	int s=0,n;
	cin>>n;
	for(int i=1;i<=n;i+=2){
		s+=i;
	}
	cout<<s;
}
/**************************************************************
	Problem: 1003
	User: wangyiyang
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/