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