int main()
{
	int a = 0, b = 0;
	scanf("%d", &a);
	int c = a;

	for (; c > 0; c--)
	{
		a = c;
		if (a % 2 == 0)
		{
			a *= -1;
		}
		b += a;
	}
	printf("%d\n", b);
	return 0;
}