def indiscriminate_time(r, tau, N_r):
    """
    Expected time spent in park by an indiscriminate poacher

    Parameters
    ----------

    r: float between 0 and 1       - the proportion of horns devalued
    tau: float                     - the time taken to kill a horn
    N_r: int                       - number of devalued rhinos that correspond
                                     to a single valued rhino
    """
    return tau * (1 - r ** N_r) / (1 - r)