Resource of type 'AWS::ElasticLoadBalancingV2::ListenerRule' with identifier 'Priority '1' is currently in use

There are so many Listener rules in my alb.My serverless’s alb priority is set up to 1.This priority has already existed in alb.So the error above occurs.

      - alb:
          priority: 1

I want this priority without hard code.Is there a way to insert rule with priority automatically?And can I set up priority with variable instead of hard code.

Since I can order the priority manually,could you add a value of priority like top in the serverless?
It means when I insert a rule,I have the highest priority.If there is a first priority,it will follow the rule I insert.