First of all I expanded the brackets as follows:

(1+x+y)(1+x+y) = 1+2x+2y+x^2+y^2+2xy
(1-x-y)(1-x-y) = 1-2x-2y+x^2+y^2+2xy

subtracting these two gives:

4x+4y = 4(x+y)

Therfore whatever integer value is chosen for x and y the answer will always be a multiple of 4.