B. Maximum Sum of Digits
greedy
You are given a positive integer \(n\).Let \(S(x)\) be sum of digits in base 10 representation of \(x\), for example, \(S(123) = 1 + 2 + 3 = 6\), \(S(0) = 0\).Your task is to find two integers \(a, b\), such that \(0 \leq a, b \leq n\), \(a + b = n\) and \(S(a) + S(b)\) is the largest possible among all such pairs.
The only line of input contains an integer \(n\) \((1 \leq n \leq 10^{12})\).
Print largest \(S(a) + S(b)\) among all pairs of integers \(a, b\), such that \(0 \leq a, b \leq n\) and \(a + b = n\).
In the first example, you can choose, for example, \(a = 17\) and \(b = 18\), so that \(S(17) + S(18) = 1 + 7 + 1 + 8 = 17\). It can be shown that it is impossible to get a larger answer.In the second test example, you can choose, for example, \(a = 5000000001\) and \(b = 4999999999\), with \(S(5000000001) + S(4999999999) = 91\). It can be shown that it is impossible to get a larger answer.