\documentclass[12pt,a4paper,sloppy]{article}
<% @latex_config={:command => 'pdflatex',:parse_twice => true} # you can override the defaults if you wish; see LatexToPdf#config %>
\usepackage{lastpage}
\usepackage{graphics}
\begin{document}

This document uses \verb|parse_twice| to calculate the last page.

Other defaults can be overridden using \verb|LatexToPdf#config| and \verb|@latex_config|.

<%= yield %>

\end{document}