def lenIfValidResult(s, charA, charB):
    last_matching_char = None
    count = 0
    for char in s:
        if char == charA or char == charB:
            if last_matching_char == char:
                return 0
            count += 1
            last_matching_char = char
    return count

def alternate(s):    
    max_len = 0
    chars_in_str = set(s)
    if len(chars_in_str) <= 1:
        return 0
    for charA in chars_in_str:
        for charB in chars_in_str:
            if charA != charB:
                result_len = lenIfValidResult(s, charA, charB)
                max_len = max(max_len, result_len)
    return max_len