def match(s1, s2):
    """
    :param s1: str, the long DNA sequence to be compared
    :param s2: str, the short DNA sequence to match
    :return: str, the most similar segment from the long DNA sequence
    """
    ans = ''
    maximum = 0
    # The maximum of match rate
    for i in range(len(s1) - len(s2) + 1):
        # (len(s1) - len(s2) + 1) is the times of matching
        match_rate = 0
        segment = s1[i: i+len(s2)]
        # The DNA segment from the long sequence to be compared
        for j in range(len(s2)):
            ch1 = segment[j]
            ch2 = s2[j]
            if ch1 == ch2:
                match_rate += 1 / len(s2)
        if maximum < match_rate:
            maximum = match_rate
            ans = segment
    return ans