def add_vp_vs(df):
    """Calculates the Vp and Vs for a las file

    Args:
        df (Pandas.DataFrame): input dataframe MUST CONTAIN `DTCO` and `DTSM`
    Returns:
        pandas.DataFrame: input dataframe with vp and vs calculated
    """
    df['Vp'] = (1000000 / df['DTCO']) / 3.281
    df['Vs'] = (1000000 / df['DTSM']) / 3.281
    df['Vp_max'] = df['Vp'].max() + 200
    return df