A12098. Maximum Sum of Digits

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.