$$
\begin{array}{l}
\text { 13. Given the function } f(x)=-2 x+4 \text {, let } \\
S_{n}=f\left(\frac{1}{n}\right)+f\left(\frac{2}{n}\right)+\cdots+f\left(\frac{n-1}{n}\right)+f(1) \\
\left(n \in \mathbf{N}_{+}\right) \text {. }
\end{array}
$$

If the inequality $\frac{a^{n}}{S_{n}}<\frac{a^{n+1}}{S_{n+1}}$ always holds, find the range of real number $a$.