template <typename Iterator, typename T>
T
parallel_accumulate(Iterator first, Iterator last, T init)
{
  unsigned long const length         = std::distance(first, last);
  unsigned long const max_chunk_size = 25;
  if (length <= max_chunk_size) {
    return std::accumulate(first, last, init);
  }
  else {
    Iterator mid_point = first;
    std::advance(mid_point, length / 2);
    std::future<T> first_half_result =
        std::async(parallel_accumulate<Iterator, T>, first, mid_point, init);
    T second_half_result = parallel_accumulate(mid_point, last, T());
    return first_half_result.get() + second_half_result;
  }
}