def hamming_no_gap(seq1, seq2):
    """Calculate pairwise Hamming distance, skipping sites with gaps."""
    n, m = 0, 0
    for c1, c2 in zip(seq1, seq2):
        if c1 != '-' and c2 != '-':
            n += 1
            if c1 != c2:
                m += 1
    return m / n