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