Sum of Squares

Write a function sum$\_$squares $(n)$ that returns the sum of the perfect squares $1^2+2^2+$cdots$+n^2$ using the formula

$S=\frac{n(n+1)(2n+1)}{6}$

Assume that $n$ is a positive integer. The return value should be an int $.($Note that the formula always evaluates to an integer. Why is that?$)$

Example:

sum$\_$squares$(200)$ returns $2686700($not $2686700\mathrm{.}0).$