D - Pairs again!!

#include <bits/stdc++.h>

using namespace std;

#define cin_2d(vec, n, m) for(int i = 0; i < n; i++) for(int j = 0; j < m && cin >> vec[i][j]; j++);
#define cout_2d(vec, n, m) for(int i = 0; i < n; i++, cout << "\n") for(int j = 0; j < m && cout << vec[i][j] << " "; j++);
#define fixed(n) fixed << setprecision(n)
#define ceil(n, m) (((n) / (m)) + ((n) % (m) ? 1 : 0))
#define fill(vec, value) memset(vec, value, sizeof(vec));
#define mul_mod(a, b, m) (((a % m) * (b % m)) % m)
#define add_mod(a, b, m) (((a % m) + (b % m)) % m)
#define all(vec) vec.begin(), vec.end()
#define rall(vec) vec.rbegin(), vec.rend()
#define sz(x) int(x.size())
#define debug(x) cout << #x << ": " << (x) << "\n";
#define fi first
#define se second
#define ll long long
#define ull unsigned long long
#define Mod  1'000'000'007
#define OO 2'000'000'000
#define EPS 1e-9
#define PI acos(-1)
template < typename T = int > using Pair = pair < T, T >;
vector < string > RET = {"NO", "YES"};

template < typename T = int > istream& operator >> (istream &in, vector < T > &v) {
    for (auto &x : v) in >> x;
    return in;
}

template < typename T = int > ostream& operator << (ostream &out, const vector < T > &v) {
    for (const T &x : v) out << x << ' ';
    return out;
}

void Solve(){
    ll n;
    cin >> n;
    ll x = -1, y = -1, curr_number = 0, sum_of_digits = 0;
    // best_answer is to get the maximum number contains 9 smaller than n
    // 0 9 99 999 9999 99999 999999  ...
    while(curr_number < n){
        x = curr_number;
        curr_number = curr_number * 10 + 9;
        // if curr_number is 9 new curr_number will be 9 * 10 = 90 + 9 = 99
    }
    // x will save the last valid number
    y = n - x;
    // get summation of digits of x and y
    while(x > 0)
        sum_of_digits += x % 10, x /= 10;
    while(y > 0)
        sum_of_digits += y % 10, y /= 10;
    cout << sum_of_digits << "\n";
}

int main(){
    ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
    int t = 1;
    //cin >> t;
    while(t--)
        Solve();
    return 0;
}