9. Using 6 different digits $a, b, c, d, e, f$ to form six three-digit numbers $\overline{a b c}, \overline{b c d}, \overline{c d e}, \overline{d e f}, \overline{e f a}, \overline{f a b}$. If $\frac{\overline{a b c}+\overline{b c d}+\overline{c d e}+\overline{d e f}+\overline{e f a}+\overline{f a b}}{3}$ is a perfect square, then the six-digit number $\overline{a b c d e f}$ is the smallest when $\qquad$
$256789$