About pointer to member function of derived class

Here's my code, and the IDE is DEV C++11

#include<iostream>
using namespace std;

class A{
    public:
        int a=15;
};
class B:public A
{

};
int main(){

    int A::*ptr=&B::a; //OK
    int B::*ptr1=&A::a; //why?
    int B::A::*ptr2=&B::a;//why?
    int B::A::*ptr3=&A::a;  //why?

} 

I have read Programming Languages — C++ and I know the type of &B::a is int A::*, but I don't realise why the next three lines will pass the compilation. And the weirdest thing to me is the syntax of int B::A::* , what's the meaning of this? I'm just a newcomer of C/C++, so please tolerate my weird question.