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