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

/**************************************************************
	Problem: 1003
	User: asd
	Language: C++
	Result: Accepted
	Time:10 ms
	Memory:2072 kb
****************************************************************/