Problem:
Score : 200 points

Problem Statement

Takahashi has two positive integers A and B.

It is known that A plus B equals N.
Find the minimum possible value of "the sum of the digits of A" plus "the sum of the digits of B" (in base 10).

Constraints

2 ≤ N ≤ 10^5

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the minimum possible value of "the sum of the digits of A" plus "the sum of the digits of B".

Sample Input 1

15

Sample Output 1

6

When A=2 and B=13, the sums of their digits are 2 and 4, which minimizes the value in question.

Sample Input 2

100000

Sample Output 2

10

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

#![allow(unused_imports)]
#![allow(non_snake_case)]

use std::cmp::{max, min, Ordering};
use std::collections::*;
use std::io::*;
use std::ops::*;
use std::*;

// -----------------------------------------------
// Framework
// -----------------------------------------------

#[allow(unused)]
fn rl() -> String {
    let mut buf = String::new();
    io::stdin().read_line(&mut buf).unwrap();
    buf.trim_right().to_owned()
}

#[allow(unused)]
fn rw<T>() -> Vec<T>
where
    T: std::str::FromStr,
    T::Err: std::fmt::Debug,
{
    let mut buf = String::new();
    io::stdin().read_line(&mut buf).unwrap();
    buf.split_whitespace()
        .map(|word| T::from_str(word).unwrap())
        .collect()
}

trait IteratorExt: Iterator + Sized {
    fn vec(self) -> Vec<Self::Item> {
        self.collect()
    }
}

impl<T: Iterator> IteratorExt for T {}

#[allow(unused)]
macro_rules! debug {
    ($($arg:expr),*) => {
        #[cfg(debug_assertions)]
        {
            let entries = &[
                $((
                    &stringify!($arg).to_string() as &fmt::Debug,
                    &($arg) as &fmt::Debug,
                )),*
            ];
            eprintln!("{:?}", DebugMap(entries));
        }
    };
}

#[allow(unused)]
struct DebugMap<'a>(&'a [(&'a fmt::Debug, &'a fmt::Debug)]);

impl<'a> std::fmt::Debug for DebugMap<'a> {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        let mut m = fmt.debug_map();
        for &(key, value) in self.0.iter() {
            m.entry(key, value);
        }
        m.finish()
    }
}

// -----------------------------------------------
// Polyfill
// -----------------------------------------------

#[derive(PartialEq, Eq, Clone, Debug)]
pub struct Rev<T>(pub T);

impl<T: PartialOrd> PartialOrd for Rev<T> {
    fn partial_cmp(&self, other: &Rev<T>) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T: Ord> Ord for Rev<T> {
    fn cmp(&self, other: &Rev<T>) -> Ordering {
        other.0.cmp(&self.0)
    }
}

#[allow(unused)]
macro_rules! eprintln {
    ($($arg:expr),*) => { _eprintln(format_args!($($arg),*)) }
}

fn _eprintln(args: fmt::Arguments) {
    let err = std::io::stderr();
    let mut err = err.lock();
    err.write_fmt(args).unwrap();
    err.write(b"\n").unwrap();
}

// -----------------------------------------------
// Solution
// -----------------------------------------------

fn g(mut n: i32) -> i32 {
    let mut sum = 0;
    while n > 0 {
        sum += n % 10;
        n /= 10;
    }
    sum
}

pub fn main() {
    let n = rl().parse::<i32>().unwrap();
    let mut min_f = std::i32::MAX;
    for a in 1..n {
        let b = n - a;
        min_f = min(min_f, g(a) + g(b));
    }
    println!("{}", min_f);
    return;
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_ok() {
        assert_eq!(7, 1 + 2 * 3);
    }
}