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