9. Half-planes $\alpha_{1}, \alpha_{2}, \ldots, \alpha_{n}$ have a common edge $l$. What is the sum of the dihedral angles $\alpha_{1} \widehat{l \alpha_{2}}, \alpha_{2} l \alpha_{3}, \ldots, \alpha_{n-1} l \alpha_{n}$, $\widehat{\alpha_{n} l} l \alpha_{1}$, which together fill the entire space?