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