A repunit is a number consisting only of the digit 1, such as 111 and 11111. Find the number of digits in the smallest repunit divisible by 21 and 9.
[10^18 - 1] / 9 ==111,111,111,111,111,111 [18-digit number]
111111111111111111 mod 9 ==0
111111111111111111 mod 21==0
