  We show that the growth rate of denominator $Q_n$ of the $n$-th convergent of
negative expansion of $x$ and the rate of approximation: $$
\frac{\log{n}}{n}\log{\left|x-\frac{P_n}{Q_n}\right|}\rightarrow
-\frac{\pi^2}{3} \quad \text{in measure.} $$ for a.e. $x$. In the course of the
proof, we reprove known inspiring results that arithmetic mean of digits of
negative continued fraction converges to 3 in measure, although the limit
inferior is 2, and the limit superior is infinite almost everywhere.
