Provision shall be made that "average final compensation" shall be the average of the highest annual regular compensations paid a member by the city during a period of five consecutive years of credited service obtained within his ten years of credited service rendered immediately preceding his retirement. If a member has less than five years of credited service, his average final compensation shall be the average of total annual regular compensations paid him by the city. In no case shall any member's average final compensation include compensation for overtime.