#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 != 0 && a*b<0)
	   a=-a;
        
   else if(abs(b)%2 == 0 && a*b>0)
	   a=-a;
	  
	   printf("%d\n",a-b);


return 0;
}