def bits_diff(bits_a, bits_b):
    """Returns the Hamming distance between two bitstrings.

    Parameters
    ----------
    bits_a : Tuple[int]
        The first string.
    bits_b : Tuple[int]
        The second string.

    Returns
    -------
    int
        The number of bits that differ.
    """

    return sum([abs(bits_a[i] - bits_b[i]) for i in range(0, len(bits_a))])