def flesch(df):
    """
    Calculates the Flesch formula for each text.
    The formula and its interpretation is given in this wiki page: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests
    
    Needed features:
    Avg_words_per_sentence
    Avg_syllables_per_word
    
    Adds column:
    Flesch - Flesch formula score for the text 
    """
    
    # Flesch formula
    df["Flesch"] = 206.835 - 1.015 * df["Avg_words_per_sentence"] - 84.6 * df["Avg_syllables_per_word"]
    
    return df