Let the vectors $\overline{\mathrm{a}},\overline{\mathrm{b}},\overline{\mathrm{c}},\overline{\mathrm{d}}$ be such that $(\overline{\mathrm{a}}\times \overline{\mathrm{b}})\times (\overline{\mathrm{c}}\times \overline{\mathrm{d}})=0$. Let ${\mathrm{\pi }}_1, {\mathrm{\pi }}_2$ be planes determined by the pairs of vectors $\overline{\mathrm{a}}, \overline{\mathrm{b}}$ and $\overline{\mathrm{c}}, \overline{\mathrm{d}}$ respectively, then angle betwen ${\mathrm{\pi }}_1$ and ${\mathrm{\pi }}_2$ is