How to count the number of pages between two labels? I tried to fined a way using this answer but not helpful.
\documentclass[12pt]{book}
\usepackage{lipsum}
\begin{document}
\lipsum[1-20]\label{start}
\lipsum[1-20]\label{end}
As mentioned in $\the\numexpr\pageref{end}-\pageref{start}$ pages ...
\end{document}