Let $\vec{a}$, $\vec{b}$ and $\vec{c}$ be three unit vectors, out of which vectors $\vec{b}$ and $\vec{c}$ are non-parallel. If $\alpha$ and $\beta$ are the angles which vector $\vec{a}$ makes with vectors $\vec{b}$ and $\vec{c}$ respectively and $\vec{a}\times \left(\vec{b}\times \vec{c}\right)=\frac{1}{2}\vec{b},$ then $\left|\alpha -\beta \right|$ is equal to