def merge_lists(a, b):
    new_list = []
    for x in a:
        new_list.append(x)
    for x in b:
        new_list.append(x)
    return new_list


def count_lower(s):
    """
    count the number of lower ascii letters in the string.

    :param s: string
    :return: int, count of lower ascii letters
    """
    list_of_lower_letters = ""
    for i in s:
        if i.islower():
            list_of_lower_letters += i
    return list_of_lower_letters

print(count_lower("ASDsdsa"))

def ends_with_pair(s):
    """
    if string ends with pair of symbols, return the index of the start of the pair.
    if not, return the index of the last symbol.
    else, return -1

    :param s: string
    :return: int, index of pair or index of single symbol or -1
    """
    if len(s) == 1:
        return 0
    elif s == "":
        return "-1"
    elif s[-1] == s[-2]:
        return len(s) - 2
    elif s[-1] != s[-2]:
        return len(s) - 1