${\overline{\mathrm{e}}}_1,{\overline{\mathrm{e}}}_2,{\overline{\mathrm{e}}}_3$ are non coplanar unit vectors and $\mathrm{\alpha }=({\overline{\mathrm{e}}}_1, {\overline{\mathrm{e}}}_2), \mathrm{\beta }=({\overline{\mathrm{e}}}_2, {\overline{\mathrm{e}}}_3), \mathrm{\gamma }=({\overline{\mathrm{e}}}_3, {\overline{\mathrm{e}}}_1)$ then the is minimum value of $\mathrm{cos\alpha } + \mathrm{cos\beta } + \mathrm{cos\gamma }$ is