MR-125 / 125. problem

Simplify the following expression:

a-1+b-1ba-1+ab-1-1+a-1+b-12-1-a-1-b-1a-1·b-1

a-1+b-1ba-1+ab-1-1+a-1+b-12-1-a-1-b-1a-1·b-1=

=ba-1+ab-1a-1+b-1+2a-1+b-1-1a-1b1ab

=ba+ab1a+1b+21a+1b-b-aab1ab

=b2+a2abb+aab+2b+aab-abb-aab

=aba2+b2aba+b+2aba+b+a-b

=aba2+b2+2ab2+aba+ba-baba+b

=aba2+b2+2ab+a+ba-baba+b

=aba2+b2+2ab+a+ba-baba+b

=a+b2+a+ba-ba+b

=a+ba+b+a-ba+b

=a+ba+b+a-ba+b

=2a