Given any positive integer x ≤ 4000, find the smallest positive integer m such that mx consists of all 9's. Return -1 if no such m exists.
11 -> 9, because 11 * 9 == 99.
12 -> -1, because 12 is even, so no multiple of it can contain only nines.
13 -> 76923, because 13 * 76923 == 999999, and no smaller positive integer, when multiplied by 13, generates an
integer containing only nines.
NOTE: Although x ≤ 4000, m can be very, very LARGE. Where necessary, the way of handling big integers appropriate to the language should be used.