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