def avi(b4, b8a):
    """
    Ashburn Vegetation Index (Ashburn, 1978).

    .. math:: AVI = 2 * b8a - b4

    :param b4: Red.
    :type b4: numpy.ndarray or float
    :param b8a: NIR narrow.
    :type b8a: numpy.ndarray or float

    :returns AVI: Index value

    .. Tip::

        Ashburn, P. 1978. The vegetative index number and crop \
        identification. The LACIE Symposium Proceedings of the \
        Technical Session, 843-855.
    """

    AVI = 2 * b8a - b4
    return AVI