def hamming_with_n(s1, s2):
    """
    Hamming Distance without counting wildcard smbols.

    Args:
        s1: the first sequence for comparison.
        s2: the second sequence for comparison.

    Returns:
        the distance without accounting unrestricted sites.
    """
    distance = 0
    for c1, c2 in zip(s1, s2):
        if c1 != c2:
            if c1 == 'N' or c2 == 'N':
                continue
            else:
                distance += 1
    return distance