#include<stdio.h>
int main()
{
	int a,b,x;
	scanf("%d %d",&a,&b);
	if (a<=0)
	{
		b=b*2;
		if (a*b>0)
		a=-a;
		printf("%d",a-b);
	}
	else 
	{
		b=-b;
		x=b;
		if (x<0)
		x=-x;
		
		if (x%2==1)
		{
			if (a*b<0)
			a=-a;
			printf("%d",a-b);
		}

		else 
		{
			if (a*b>0)
			a=-a;
			printf("%d",a-b);
		}
	}
}