Negative Directional Index (-DI).

    Formula:
    -DI = 100 * SMMA(-DM) / ATR

def negative_directional_index(close_data, high_data, low_data, period):
    """
    Negative Directional Index (-DI).

    Formula:
    -DI = 100 * SMMA(-DM) / ATR
    """
    catch_errors.check_for_input_len_diff(close_data, high_data, low_data)
    ndi = (100 *
           smma(negative_directional_movement(high_data, low_data), period) /
           atr(close_data, period)
           )
    return ndi