
\documentclass{article}
\usepackage{oubraces}
\pagestyle{empty}
\begin{document}
\[
\overunderbraces{&\br{2}{x}& &\br{2}{y}}%
  {a + b +&c + d +&e + f&+&g + h&+ i + j&+ k + l}%
  {& &\br{3}{z}}
= \pi r^2
\]
\end{document}
