( b) In the event of the Secretary-General' s retirement before the completion of the full term of his appointment, he shall be provided with a retirement allowance equal to one half of the full allowance if he has served at least one year, but less than two years; for longer periods, the retirement allowance shall increase by one eighth of the full allowance for each completed year of service beyond one year until reacbing its full level on the completion of five years of service;