If S={θ ∈[-π, π]: cos θ cos (5 θ)/2= cos 7 θ cos (7 θ)/2}, then n(S) is equal to ____.

If $\displaystyle S=\left\{\theta \in[-\pi, \pi]: \cos \theta \cos \frac{5 \theta}{2}=\cos 7 \theta \cos \frac{7 \theta}{2}\right\}$, then $\displaystyle n(S)$ is equal to $\displaystyle \_\_\_\_$.