99 - ZOJ Monthly, November 2010 - J
The digital difference of a positive number is constituted by the difference between each two neighboring digits (with the leading zeros omitted). For example the digital difference of 1135 is 022 = 22. The repeated digital difference, or differential root, can be obtained by caculating the digital difference until a single-digit number is reached. A number whose differential root is 7 is also called July Number. Your job is to tell how many July Numbers are there lying in the given interval [a, b].
There are multiple cases. Each case contains two integers a and b. 1 ≤ a ≤ b ≤ 109.
One integer k, the number of July Numbers.
Author: HE, Ningxu
Contest: ZOJ Monthly, November 2010