def geo_mean_return(returns):
    """
    calculate the geometric mean return of a pandas time series.  please note
    na's are dropped so errors will not be returned
    :param returns: the time series
    :return: the geomtreic mean return
    """
    cr = ((1 + returns).cumprod())[-1]
    gr = (cr ** (1 / len(returns))) - 1
    return gr