It freaks me out!

If $S$ is the sum of all positive integers satisfying $x$ in the equation below:

$x^{2} + (x+1)^{2} + (x+2)^{2}+ \cdots +(x+500)^{2} = (x+501)^{2} + (x+502)^{2} +\cdots + (x+1000)^{2}$

Find $S \bmod 9$ .