IMSO 2017 — Mathematics — Short Answer Question 15

If $\overline{abcdef}$ represents a 6-digit number, where $a$, $b$, $c$, $d$, $e$ and $f$ denote distinct digits, what is the largest possible value of $\overline{abc}+\overline{bcd}+\overline{cde}+\overline{def}$?