NO-ABEL 2008 Problem 1

Let $s(n) = \frac16 n^3 - \frac12 n^2 + \frac13 n$ . (a) Show that $s(n)$ is an integer whenever $n$ is an integer. (b) How many integers $n$ with $0 < n \le 2008$ are such that $s(n)$ is divisible by $4$ ?