#include<stdio.h>
#include<math.h>
int main()
{
	double a,b;
	scanf("%lf%lf",&a,&b);
	if(a > 0)
	{
		b = -b;
	}
	else
		b = b * 2.0;
	int k = fabs(b);
	if(k % 2 != 0)
	{
		if(b < 0)
		{
			if(a < 0)
				a = a;
			else if(a > 0)
				a = -a;
		}
		else if(b > 0)
		{
			if(a < 0)
				a = -a;
			else if(a > 0)
				a = a;
		}
		else if(b = 0)
			a = 0;
	}
	else if(k % 2 == 0)
	{
		if(b < 0)
		{
			if(a < 0)
				a = -a;
			else if(a > 0)
				a = a;
		}
		else if(b > 0)
		{
			if(a < 0)
				a = a;
			else if(a > 0)
				a = -a;
		}
		else if(b = 0)
			a = 0;
	}
	printf("%.0f\n",a-b);
}