Casting b/w Base Derived Class

[usman999_1]

Hi *!
Lets say i have a base class & derived classes like

class main {
  public:
  virtual void foo(){}
 };

class derive1  { void foo(){}};
class derive2 : public main, public derive1 { void foo(){}}; 

Now if i do something like that

DWORD ret = (DWORD)static_cast<main*>((derive2*)1) - 1 ; // now ret = 0    O.K
ret = (DWORD)static_cast<derive1*>((derive2*)1) - 1 ;  // now ret = 4 WHY????????? & HOW?????? 

What I dont understand is (it has to do something with virtual functions i think) But what exactly is it?????????
and secondly why am i getting 4 just by casting as far i understand casting is a hint that the var that is being casted to be treated of a different type then it is now after the cast but doenst have anything to do with the value.
Thanks for your time,
Regards,
Usman.