#include <cstdint>
#include <iterator>
#include <type_traits>

template<typename RandomAccessIterator>
auto average(RandomAccessIterator first, RandomAccessIterator last)
    -> long double
{
    static_assert(std::is_integral<
        typename std::iterator_traits<RandomAccessIterator>::value_type
    >::value, "average only available for built-in integer types");

    auto size = std::distance(first, last);
    if (size == 0) return 0.0L;

    std::intmax_t accumulator = 0;
    long double res = 0.0L;

    while (first != last) {
        std::intmax_t tmp;
        if (__builtin_add_overflow(*first, accumulator, &tmp)) {
            res += static_cast<long double>(accumulator) / static_cast<long double>(size);
            accumulator = *first;
        } else {
            accumulator = tmp;
        }
        ++first;
    }

    res += static_cast<long double>(accumulator) / static_cast<long double>(size);
    return res;
}