(x+y)^2+(x+m)^2-(m+n)^2-(y+n)2
=(x+y)^2-(m+n)^2+(x+m)^2-(y+n)2
=(x+y+m+n)(x+y-m-n)+(x+m+y+n)(x+m-y-n)
=(x+y+m+n)(x+y-m-n+x+m-y-n)
=2(x-n)(x+y+m+n)