Let $\mathcal{O}$ be the ring of integers in an algebraic number field and let $R \subset \mathcal{O}$ be an order.  For instance, we might have $\mathcal{O} = \{\text{$x+i y$ $|$ $x,y \in \mathbb{Z}$}\}$ and $R = \{\text{$x+i y \in \mathcal{O}$ $|$ $y$ even}\}$. Question : Is $\text{SL}_n(R)$ a finite-index subgroup in $\text{SL}_n(\mathcal{O})$?  I might be missing something obvious here, but I'm having trouble proving this (maybe because it isn't true?).