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