===== PascalABC.NET ======

begin

 var (k, m) := ReadInteger2;

 var f: integer;

 if k > m div 2 then

   f := k * m - 4

 else if (k <= m div 2) and (k > 0) then

   f := k - 4 * m

 else

     f := m - 4;

 Print(f)    

end.