#include<stdio.h>
#include<math.h>

int main()
{
    int a, b;
	scanf("%d %d", &a, &b);
	if (a > 0)
	{
		b = -b;
	}
	else
	{
		b *= 2;
	}
	if (abs(b) % 2 == 1)
	{
		if ((a < 0 && a * b < 0) || (a > 0 && a * b < 0))
			a *= -1;
	}
	if (abs(b) % 2 == 0)
	{
		if ((a < 0 && a * b > 0 )|| (a > 0 && a * b > 0))
			a *= -1;
	}
	printf("%d", a - b);
	return 0;
}