def _get_kdjj(df, n_days):
        """ Get the J of KDJ

        J = 3K-2D
        :param df: data
        :param n_days: calculation range
        :return: None
        """
        k_column = 'kdjk_{}'.format(n_days)
        d_column = 'kdjd_{}'.format(n_days)
        j_column = 'kdjj_{}'.format(n_days)
        df[j_column] = 3 * df[k_column] - 2 * df[d_column]