C-Code

#include <iostream>
#include <string>
using namespace std;

int get_number(const int&, const int&);

int main()
{
    cout << get_number(245, 12) << endl;
    cout << get_number(245, 100) << endl;
    cout << get_number(245, 13'000) << endl;
}

int get_number(const int& value, const int& nth)
{
    const auto& lengthZeros{ (int)log10(nth) };
    const auto& leadingZeros{ string(lengthZeros, '0') };
    auto subtrahend{ 1 };
    auto length{ 0 };
    auto exponent{ 0 };

    if (nth == 0 || nth == 1) return value;

    while (true) // Subtrahenden ermitteln: 2, 12, 112 ...
    {
        auto power = (int)pow(10, exponent);
        if (nth >= subtrahend + power)
        {
            subtrahend += power;
            length++;
        }
        else break;
        exponent++;
    }
    const auto& l{ (leadingZeros + to_string(nth - subtrahend)) };
    return stoi(to_string(value) + l.substr(l.length() - length, length));
}