Let $\overrightarrow{a},\overline{b}$ and $\overline{c}$  be three unit vectors, out of which 

vectors $\overrightarrow{b}$and $\overrightarrow{c}$  are non-parallel. If $a$ and $\beta$ are the

angles which vector $\overline{a}$ makes with vectors $\overrightarrow{b}$ and $\overline{c}$

respectively and $\overrightarrow{a}\times (\overrightarrow{b}\times \overrightarrow{c})=\frac{1}{2}\overrightarrow{b}$, then $|\alpha -\beta |$ is 

equal to