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