# How to put a box in aligned equations?

Can someone help me with the following question?

I'm writing an equation like this:

\begin{align*}
a =& b - c \\
-& d - e
\end{align*}


How can I put a box around the part

b - c \\
d - e


Thanks.

\documentclass{article}
$\begin{array}{r@{}|l|} \cline{2-2} a =& b - c \\ -& d - e\\ \cline{2-2} \end{array}$