typename std::iterator_traits<It>::difference_type 
    do_distance(It first, It last, std::input_iterator_tag)
{
    typename std::iterator_traits<It>::difference_type result = 0;
    while (first != last) {
        ++first;
        ++result;
    }
    return result;
}

template<class InputIt, class UnaryPredicate>
typename iterator_traits<InputIt>::difference_type
    count_if(InputIt first, InputIt last, UnaryPredicate p)
{
    typename iterator_traits<InputIt>::difference_type ret = 0;
    for (; first != last; ++first) {
        if (p(*first)) {
            ret++;
        }
    }
    return ret;
}