VMO 1998 Problem 3

The sequence $a_0$, $a_1$, $a_2$, ... is defined by $a_0$= 20, $a_1$ = 100, $a_{n+2}$ = $4a_{n+1}$ + $5a_n$ + 20. Find the smallest m such that $a_m$ - $a_0$, $a_{m+1}$ - $a_1$, $a_{m+2}$ - $a_2$, ... are all divisible by 1998.