Let
$$
A=\{1,2,\ldots,p-1\},\qquad B=\{1,2,\ldots,q-1\}
$$
where $p,q$ are primes not necessarily distinct.
Is there any elementary way to find the cardinal of the following set 
$$
AB=\{ab:\ a\in A,\ b\in B\}
$$
and also the form of them.
Thanks!