Untitled

#include <iostream>
using namespace std;

int main()
{
    int a, b, c;
    cin >> a >> b;

    if (a % 2 != 0 or b % 2 == 0 or b > 0 or a < 0)
    {
        cout << "NO";
    }
    else if (a % 2 == 0 and b % 2 != 2)
    {
        cout << a - b;
    }

}