What is the sum of [1 / 2XY + Y2 + 1] + [x2-1 / 2xy-2y2-1]

What is the sum of [1 / 2XY + Y2 + 1] + [x2-1 / 2xy-2y2-1]

【1/2xy+y2+1】+【x2-1/2xy-2y2-1】
=1/2xy+y²+1+x²-1/2xy-2y²-1
=-y²+x²