(m-n)^2+2(m+n)(m-n)+(m-n)^2=(m-n)^2+2(m+n)(m-n)-(m+n)^2=((m-n)-(m+n))^2=(m-n-m-n)^2=(-2n)^2=2n^2=4002^2
(m-n)^2+2(m+n)(m-n)+(m-n)^2=(m-n)^2+2(m+n)(m-n)-(m+n)^2=((m-n)-(m+n))^2=(m-n-m-n)^2=(-2n)^2=2n^2=4002^2