def derive_bt_mac(base_mac,offset):
    """
    Derives the BT_MAC from the BASE_MAC and the BT_MAC LSB offset.
    """
    base_mac_lsb=int(str(base_mac[-1]), base=16)+offset
    base_mac[-1]=format(base_mac_lsb, 'x')
    bt_mac='-'.join(base_mac)
    return bt_mac.strip()