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)

macro_rules! input {
     (source = $s:expr, $($r:tt)*) => {
         let mut iter = $s.split_whitespace();
         input_inner!{iter, $($r)*}
     };
     ($($r:tt)*) => {
         let s = {
             use std::io::Read;
         let mut s = String::new();
         std::io::stdin().read_to_string(&mut s).unwrap();
         s
     };
     let mut iter = s.split_whitespace();
     input_inner!{iter, $($r)*}
     };
}
     
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
 
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}
 
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
 
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
 
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
 
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
 
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_imports)]
use std::io::{stdout, Write, BufWriter};
#[allow(unused_imports)]
use std::cmp::{max, min, Ordering};
//#[allow(unused_imports)]
//use proconio::marker::{Isize1, Usize1, Bytes, Chars};

//#[proconio::fastout]
fn main() {
    input!{
        n: i64,
    }
    let mut order = 0;
    for i in 1.. {
        if 10i64.pow(i) > n {
        order = i - 1;
        break;
        }
    }
    if order == 0 { println!("{}", n); }
    else {
        for i in 0..10 {
            if (10 - i) * 10i64.pow(order) - 1 <= n {
                println!("{}", 9 * (order + 1) - i as u32);
                break;
            }
        }
    }
}