In the question, there is not defined what should be the result for
the case of A=0 and B=0.
I initially suppose that the result is 0 and got WA all the time,
trying desperately to find an error in other parts of the program.
When I already decided that I give a last try and then leave this
program, I tried the result 1 for this case and surprizingly my
program was Accepted.

This case should be described in the question it self!!!

The result for A=0 and B=0 is '1'.

It is right.

It just like you put nothing into the box.