def s2rep(b4, b5, b6, b7):
    """
    Sentinel-2 Red-edge Position (Frampton et al., 2013).

    .. math:: S2REP = 700 + 35*((((b7 - b4/2) - b5)/b6-b5))

    :param b4: Red.
    :type b4: numpy.ndarray or float
    :param b5: Red-edge 1.
    :type b5: numpy.ndarray or float
    :param b6: Red-edge 2.
    :type b6: numpy.ndarray or float
    :param b7: Red-edge 3.
    :type b7: numpy.ndarray or float

    :returns S2REP: Index value

    .. Tip::

        Frampton, W.J., Dash, J., Watmough, G., Milton, E.J. 2013. \
        Evaluating the capabilities of Sentinel-2 for quantitative \
        estimation of biophysical variables in vegetation. ISPRS Journal \
        of Photogrammetry and Remote Sensing 82, 83-92. \
        doi:10.1016/j.isprsjprs.2013.04.007.
    """

    S2REP = 700 + 35*((((b7 - b4/2) - b5)/b6-b5))
    return S2REP