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