For any positive, base-10 integer N, define f(N) as the number of times you have to add up its digits until you get a one-digit number. For example, f(51) = 1 because 5+1 = 6, a one-digit number. Meanwhile, f(98) = 2, since 8+9 = 17, a two-digit number, and then adding up those digits gives you 1+7 = 8, a one-digit number.
Find the smallest whole number N such that f(N) = 4