For natural numbers n, the digital sum S(n) denotes the sum of the
digits in n. Moreover, the notation Sp(n) stands for the sum of the digital
sums of the prime factors of n, counting multiplicity. We call a composite
n a Smith number when S(n) = Sp(n). For example, the number 728 is
Smith because S(728) = 7+2+8 = 17 and, since 728 = 222713,
we also have Sp(728) = 2 + 2 + 2 + 7 + 1 + 3 = 17.