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