$$(a+b-2)(a+b)-(a-b)^{2}+1$$
$\left(2a-1\right)\left(2b-1\right)$
$$a^{2}+2ab+b^{2}-2a-2b-\left(a-b\right)^{2}+1$$
$$a^{2}+2ab+b^{2}-2a-2b-\left(a^{2}-2ab+b^{2}\right)+1$$
$$a^{2}+2ab+b^{2}-2a-2b-a^{2}+2ab-b^{2}+1$$
$$2ab+b^{2}-2a-2b+2ab-b^{2}+1$$
$$4ab+b^{2}-2a-2b-b^{2}+1$$
$$4ab-2a-2b+1$$
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$4ab-2a-2b+1$