template<typename Iterator,typename T>
T parallel_accumulate(Iterator first,Iterator last,T init)
{
    unsigned long const length=std::distance(first,last);

    if(!length)
        return init;

    unsigned long const block_size=25;
    unsigned long const num_blocks=(length+block_size-1)/block_size;

    std::vector<std::future<T> > futures(num_blocks-1);
    thread_pool pool;

    Iterator block_start=first;
    for(unsigned long i=0;i<(num_threads-1);++i)
    {
        Iterator block_end=block_start;
        std::advance(block_end,block_size);
        futures[i]=pool.submit(accumulate_block<Iterator,T>());
        block_start=block_end;
    }
    T last_result=accumulate_block()(block_start,last);
    T result=init;
    for(unsigned long i=0;i<(num_blocks-1);++i)
    {
        result+=futures[i].get();
    }
    result += last_result;
    return result;
}