def get_gb_acc_id(year, case_index):
    """
    Returns global accident id for GB, year and index of accident.

    GB ids will be in form 2200700000001234
    2 at the beginning means it is from Great Britain
    2007 means the year
    1234 means the case ID as in original data
    """
    try:
        acc_id = 2000000000000000
        acc_id += year * 100000000000
        acc_id += case_index
    except KeyError:
        raise ValueError("Country code incorrect")
    return acc_id