def common_start(first_number, last_number):
    """ Get the common starting characters of the two strings first_number and last_number """
    str_first = str(first_number)
    str_last = str(last_number)
    common = ''
    for a, b in zip(list(str_first), list(str_last)):
        if a == b:
            common = common + a
        else:
            return common
    return None
