(a-b+c-d)(a+b-c-d)
=[(a-d)+(c-b)][(a-d)-(c-b)]
=(a-d)^2-(c-b)^2
=a^2-2ad+d^2-(c^2-2bc+b^2)
=a^2+d^2-2ad-c^2-b^2+2bc