The principal value of $\cos^{-1}\bigg(-\Large {\frac{1}{2}}\normalsize \bigg)$ is_____________.
$\begin{array}{1 1} \frac{-2\pi}{3} \\ \frac{-\pi}{3} \\ \frac{\pi}{3} \\ \frac{2\pi}{3} \end{array}$