def hamming_distance(s1, s2):
    """
    calculate hamming distance between two strings (must be same length)
    :param s1: first string
    :param s2: second string
    :return: number of mismatches between strings (if not same length, returns 100)
    """
    if len(s1) != len(s2):
        return 100
    else:
        return sum(ch1 != ch2 for ch1, ch2 in zip(s1, s2))