def transform_df_to_numbers(df):
    """
    Subsitutes difficulties with numbers to work with them in a better way, from 1 to 3

    :param df: Dataframe to transform to numbers to
    :return transformed datafarme

    """

    mapping = {'LOW': 1, 'MEDIUM': 2, 'HIGH': 3, 'undef': -1}
    df = df.replace({'physDifficulty': mapping, 'psyStress': mapping, 'psyDifficulty': mapping})

    for col in ['physDifficulty', 'psyStress', 'psyDifficulty']:
        df[col] = df[col].astype('int64')
    return df