22.28*. What is the maximum number of acute angles in a non-convex $n$-gon

保留源文本的换行和格式，翻译结果如下：

22.28*. What is the maximum number of acute angles in a non-convex $n$-gon
22.28. Let $k$ be the number of acute angles in an $n$-sided polygon. Then the sum of its angles is less than $k \cdot 90^{\circ} + (n-k) \cdot 360^{\circ}$. On the other hand, the sum of the angles of an $n$-sided polygon is $(n-2) \cdot 180^{\circ}$ (see problem 22.24), so $k \cdot 90^{\circ} + (n-k) \cdot 360^{\circ} > (n-2) \cdot 180^{\circ}$, i.e., $3 k < 2 n + 4$. Therefore, $k \leqslant [2 n / 3] + 1$, where $[x]$ denotes the greatest integer not exceeding $x$.

Examples of $n$-sided polygons having $[2 n / 3] + 1$ acute angles are shown in Fig. 22.15.