#include<bits/stdc++.h>
using namespace std;
int a,b;

int main(){
	cin>>a>>b;
	if(a > 0) b = -b;
	else b = 2*b;
	if(abs(b)%2 == 1 && a*b < 0){
		a = -a;
	}else if(abs(b)%2 == 0 && b == 0 && a > 0){
		a = -a;
	}else if(abs(b)%2 == 0 && a*b > 0){
		a = -a;
	}
	cout<<a-b<<endl;
	return 0;
}