If $\overline{\mathrm{AB}} = 3\overline{\mathrm{p}} - \overline{\mathrm{q}}$ and $\overline{\mathrm{AD}} = \overline{\mathrm{p}} + 3\overline{\mathrm{q}}$ are the adjacent sides of a parallelogram ABCD where $\left|\overline{\mathrm{p}}\right|= 2 =\left|\overline{\mathrm{q}}\right|, (\overline{\mathrm{p}},\overline{\mathrm{q}}) =\frac{\mathrm{\pi }}{3},$ then the length of the diagonal $\overline{\mathrm{AC}}$ is