Задача числа

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main()
{
    long long n;
    cin >> n;
    if (n < 10)
    {
        cout << n;
        return 0;
    }
    vector<int> d;
    while (n > 0)
    {
        d.push_back(n % 10);
        n /= 10;
    }
    reverse(d.begin(), d.end());
    long long answer = 0;

    vector<long long> c(d.size());
    c[0] = 1;
    c[1] = 10;
    for (int i = 2; i < (int)c.size(); i++)
    {
        c[i] = 10 * c[i - 2];
    }
    answer += 9;

    for (int len = 2; len < (int)d.size(); len++)
    {
        answer += 9 * c[len - 2];
    }

    if (d[0] > 1)  answer += (d[0] - 1) * c[d.size() - 2];

    for (int i = 1; i < (int)d.size() / 2; i++)
    {
        if (d.size() - i - i - 2 < 0)throw 1;
        answer += d[i] * c[d.size() - i - i - 2];
    }

    if (d.size() % 2 == 1)  answer += d[d.size() / 2];

    vector<int> r = d;
    for (int i = 0; i < (int)r.size() / 2; i++)   r[r.size() - 1 - i] = r[i];

    bool le = true;
    for (int i = 0; i < (int)d.size(); i++)
    {
        if (r[i] < d[i])   break;
        if (r[i] > d[i]) {
            le = false;
            break;
        }
    }
    if (le) answer++;
    cout << answer << endl;
}