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

return 0;
}