#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstring>
#include<string>
#include<iostream>
#include<map>
#include<vector>
#include<set>
#include<queue>
using namespace std;

int a, b;

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