#include<iostream>
using namespace std;
#define lpi(m,n,b) for(int i = m;i <= n;i += b)
int main()
{
	int n,s = 0,t = 1;
	cin >> n;
	lpi(1,n,1)
	{
		s += i * t;
		t *= -1;
	}
	cout << s;
 }