#include <algorithm>
#include <fstream>
#include <iostream>
#include <iterator>
#include <numeric>
#include <set>
#include <vector>

std::set<int> read_file(std::string const& file)
{
    std::ifstream ifs { file };
    return { std::istream_iterator<int> { ifs },
             std::istream_iterator<int> { } };
}

int main()
{
    auto receipt = read_file("receipt.txt");
    auto payments = read_file("payments.txt");

    auto total_out = std::accumulate(std::begin(receipt), std::end(receipt), 0);
    auto total_in  = std::accumulate(std::begin(payments), std::end(payments), 0);

    if (total_out != total_in)
    {
        std::cout << "Payments doesn't match total cost" << std::endl;
    }

    std::cout << "Incorrect payments:" << std::endl;
    std::set_difference(std::begin(payments), std::end(payments),
                        std::begin(receipt), std::end(receipt),
                        std::ostream_iterator<int>(std::cout, "\n"));
}