9. The curvature of a solid figure is $2 \pi$ minus the sum of the dihedral angles (the interior angles of the faces of a polyhedron). For example, for a cube, the dihedral angle is $2 \pi - 3 \times \frac{\pi}{2} = \frac{\pi}{2}$, so its total curvature is $\frac{\pi}{2} \times 8 = 4 \pi$. The total curvature of a square pyramid (4 triangles and one square) is $\qquad$.
【Answer】 $4 \pi$