#include <thread>
#include <mutex>
#include <vector>
#include <iostream>

using It = std::vector<int>::iterator;

void task(It begin, It end, int& total)
{
    int result { 0 };
    for (It it { begin }; it != end; ++it)
        result += *it;
    total += result;
}

int sum(std::size_t thread_count, std::vector<int>& values)
{
    std::size_t const size { values.size() / thread_count };
    std::vector<std::thread> pool { };
    int result { };

    for (std::size_t i { 0 }; i < thread_count; ++i)
    {
        auto begin { values.begin() + size * i };
        auto end   { values.begin() + size * (i + 1) };
        pool.emplace_back(task, begin, end, std::ref(result));
    }

    for (std::thread& thread : pool)
        thread.join();

    return result;
}

int main()
{
    std::vector<int> v(1000000, 1);
    int answer { sum(20000, v) };

    std::cout << answer << std::endl;
}