IMSO 2010 — International Mathematics — Short Answer Question 19

For a positive integer $n$, let $d(n)$ be its digit sum. For example, $d(123)=1+2+3=6$ and $d(7879)=7+8+7+9=31$. Find $d(d(999\,888\,777\,666\,555\,444\,333\,222\,111))$.