def pd_rolling_mean(series, window_size):
    """
    Compute rolling mean on a Series 
    with the given window_size
    and return only non-None rows,
    i.e. starning from row number window_size - 1
    and until the end. 
    """
    
    rolling_mean = series.rolling(window_size).mean()
    
    return rolling_mean[window_size - 1:]