def _get_wr(cls, df, n_days):
        """ Williams Overbought/Oversold Index

        WMS=[(Hn—Ct)/(Hn—Ln)] ×100
        Ct - the close price
        Hn - N days high
        Ln - N days low

        :param df: data
        :param n_days: N days
        :return: None
        """
        n_days = int(n_days)
        ln = df['low'].rolling(min_periods=1, window=n_days,
                               center=False).min()

        hn = df['high'].rolling(min_periods=1, window=n_days,
                                center=False).max()
        column_name = 'wr_{}'.format(n_days)
        df[column_name] = (hn - df['close']) / (hn - ln) * 100