Sum intervals

sum_intervals.py

def sum_intervals(interval_list):
    interval_list.sort(key=lambda lst: lst[0])

    # sum numbers in the interval with the lowest begin number
    begin, end = interval_list[0]
    max = end
    result = sum(range(begin, end + 1))

    # sum numbers from the remaining intervals
    for begin, end in interval_list[1:]:
        if begin <= max:
            result += sum(range(max + 1, end + 1))
        if begin > max:
            result += sum(range(begin, end + 1))

        max = end

    return result