def lab_monthly(dataframe):
    """
    Extract monthly data from a labstat file

    Parameters:
        dataframe (dataframe): A dataframe containing labstat data

    Returns:
        dataframe
    """
    dataframe['month'] = dataframe['period'].str[1:].copy().astype(int)

    dataframe['value'] = dataframe['value'].astype(int)

    dataframe = dataframe.loc[~dataframe['month'].isin([13])]

    return dataframe