Let $\overrightarrow{\mathrm{a}}\text{ and }\overrightarrow{\mathrm{b}}$be two unit vectors such that the angle between them is $\frac{\mathrm{\pi }}{3}$. If $\mathrm{\lambda }\overrightarrow{\mathrm{a}}+2\overrightarrow{\mathrm{b}}\text{ and }3\overrightarrow{\mathrm{a}}-\mathrm{\lambda }\overrightarrow{\mathrm{b}}$ are perpendicular to each other, then the number of values of $\mathrm{\lambda }$ in [–1, 3] is :