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