1060B · Maximum Sum of Digits

Problem: 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.

Input Format: The only line of input contains an integer $n$ $(1 \leq n \leq 10^{12})$.

Output Format: Print largest $S(a) + S(b)$ among all pairs of integers $a, b$, such that $0 \leq a, b \leq n$ and $a + b = n$.

Note: 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.