Maximum Sum of Digits

记 $S(x)=$ $x\text{的各个数位之和}$。 例如：$S(123)=1+2+3=6$，$S(0)=0$。 给定整数 $n\ \left(1 \le n \le 10^{12}\right)$ ，求一对自然数 $a, b\ (0 \le a, b \le n)$，使得 $S(a)+S(b)$ 最大。输出这个最大值。

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 $ .

35

17

10000000000

91

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.