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