#include<bits/stdc++.h>
using namespace std;
long int n;
long int ans;
int main(){
	cin>>n;
	for(int i=1;i<=n;i+=2){
		ans+=i;
	}
	cout<<ans<<endl;
	return 0;
}
/**************************************************************
	Problem: 1003
	User: zengdongxin
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/