The density function for the number of times the riders scream on a roller coaster is given by $$f(x) = \begin{cases} \frac{1}{3 \pi}( 1 - \cos(6 x)) & \text{if} \;\; 0\leq x \leq 3 \pi\\ 0 & \text{otherwise.} \end{cases}$$ Find the mean number of screams over the course of the ride.