#include<stdio.h>
#include<math.h>

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