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