#include<bits/stdc++.h>
using namespace std;
int main()
{
	long long n,sum = 0;
	cin >> n;
	for( int i = 1; i <= n; i+=2)
	{
		sum += i;
	}
	cout << sum;
 } 
/**************************************************************
	Problem: 1003
	User: yangwanning
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/