9. For the four-digit number $\overline{a b c d}$, its reverse is $\overline{d c b a}$. When these two four-digit numbers are added together, the sum is a five-digit number $\overline{1 x 2 x 3}$, where the tens and thousands digits of this five-digit number are the same. What is the smallest four-digit number $\overline{a b c d}$? $\qquad$
$4029$