Арифметическая прогрессия в массиве - C++

#include <iostream>
 
template <typename T, typename Iterator>
T difference(Iterator first, Iterator last)
{
    if (first == last)
        return 0;
    if (first + 2 == last)
        return *(first + 1) - *first;
    Iterator next = first + 1;
    T diff = *next - *first;
    while (next != last)
        if (*next++ - *first++ != diff)
            return 0;
    return diff;
}
 
int main()
{
    int a[] = { 1, 3, 5, 7, 9, 11, 13 };
    std::cout << difference<int>(std::begin(a), std::end(a)) << std::endl;
    return 0;
}