def complex_words_pct(df):
    """
    Get percentage of complex words as defined by Gunning.
    Complex words (or polysyllables) are those with three or more syllables.
    
    Needs features:
    N_polysyllables
    N_words
    
    Adds features:
    Complex_word_percent: percentage of complex words (Gunning)
    
    :param: the dataframe with the dataset
    :returns: the dataframe with the added feature
    """
     
    # get percentage
    df["Complex_word_percent"] = df["N_polysyllables"] / df["N_words"]
    
    return df