Problem:
Score : 300 points

Problem Statement

Find the maximum possible sum of the digits (in base 10) of a positive integer not greater than N.

Constraints

1\leq N \leq 10^{16}

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the maximum possible sum of the digits (in base 10) of a positive integer not greater than N.

Sample Input 1

100

Sample Output 1

18

For example, the sum of the digits in 99 is 18, which turns out to be the maximum value.

Sample Input 2

9995

Sample Output 2

35

For example, the sum of the digits in 9989 is 35, which turns out to be the maximum value.

Sample Input 3

3141592653589793

Sample Output 3

137

Solution in Rust (1.15.1) (judge verdict: Accepted)

fn main() {
    let s = read_line();
    let n = s.trim();
    let (fst, snd) = n.split_at(1);
    let fst: i32 = fst.parse().unwrap();
    let len = n.len() as i32;
    if len == 1 {
        println!("{}", fst);
    } else if String::from(snd) == std::iter::repeat("9").take(n.len() - 1).collect::<String>() {
        println!("{}", fst + (len - 1) * 9);
    } else {
        println!("{}", (fst - 1) + (len - 1) * 9);
    }
}

fn read_line() -> String {
    let stdin = std::io::stdin();
    let mut buf = String::new();
    stdin.read_line(&mut buf).unwrap();
    buf
}