Divisible by 45 (JA3-Task-1-Big-Division) C++

#include <iostream>
#include <algorithm>
#include "BigInt.h"

using namespace std;

unsigned short sumDigits(const BigInt& number)
{
    unsigned short sum = 0;
    for (const auto &ch : number.getDigits())
        sum += ch - '0';
    return sum;
}

int main()
{
    string szInput;
    cin >> szInput;
    BigInt start(szInput);
    cin >> szInput;
    BigInt end(szInput);

    unsigned short remainder9 = (9 - sumDigits(start) % 9) % 9;
    unsigned short remainder5 = (start.getDigits().back() - '0' + remainder9) % 5;
    unsigned short remainder45 = 9 * remainder5 + remainder9;
    BigInt current = start + remainder45;
    while(current < end)
    {
        cout << current << endl;
        current += 45;
    }

    return 0;
}