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;
    }
}