def ratings_std(df):
    """calculate standard deviation of ratings from the given dataframe
    parameters
    ----------
    df (pandas dataframe): a dataframe cotanis all ratings 

    Returns
    -------
    standard deviation(float): standard deviation of ratings, keep 4 decimal
    """
    std_value = df['ratings'].std()
    std_value = round(std_value,4) 
    return std_value