[Tex/LaTex] Complex fractions

Question

How to make this more adequate ('beatifuller' :))? 2 needs to be better shown as being in the power of n-1, but the current representation just smothers it.

\frac{1}{1-\sum_{2}^{n} \frac{1}{{\frac{r}{R}}^(2^(n-1))}}

\documentclass[10pt]{article}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{textcomp}
\begin{document}
\frac{1}{1-\sum_{2}^{n} \frac{1}{{\frac{r}{R}}^(2^(n-1))}}
\end{document}

Answer 1

one more variation:

\documentclass[10pt]{article}
\usepackage{nccmath}

\begin{document}

\[
\frac{1}{1-\sum\limits_{2}^{n} \Bigl(\mfrac{r}{R}\Bigr)^{\! -2^{n-1}}}
\]

\end{document}