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