  Let G be an even orthogonal or unitary group over a number field. Based on
the same observation used in arXiv:1705.10106, we prove the Arthur's
multiplicity formula for the generic part of the automorphic discrete spectrum
of G by using the theta lift. We also consider a class of non-generic
A-parameters and obtain a multiplicity formula in this case. In particular, we
obtain a description of the full automorphic discrete spectrum of even
orthogonal or unitary groups with Witt index less or equal to one.
