int main()
{
	int l, k;
	int count;
	scanf("%d%d", &l, &k);
	
	if (l % (2 * k) == 0)
		count = l / (2 * k) - 1;
	else
		count = l / (2 * k);
	
	printf("%d\n", k*count);

	return 0;
}