If $ \vec{a}, \vec{b}, \vec{c} $ are three unit vectors such that $ \vec{a} \times (\vec{b} \times \vec{c}) = \frac{\sqrt{3}}{2} \vec{b} + \frac{1}{2} \vec{c} $, and $ \alpha, \beta $ are the angles between $ \vec{a}, \vec{c} $ and $ \vec{a}, \vec{b} $ respectively, then $ \alpha + \beta = ? $