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