2m^2 n^2-2n^2+(mn-1)(m-1)^2
=2n^2(m^2-1)+(mn-1)(m-1)^2
=2n^2(m-1)(m+1)+(mn-1)(m-1)^2
=(m-1)[2n^2(m+1)+(mn-1)(m-1)]
=(m-1)(2n^2m+2n^2+m^2n-m-mn+1)