firestore rule : allow read, write only admin

firestoreRule.txt

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /chats/{userId}/{document=**} {
      allow read, write: if
      
      (
      
              userId == request.auth.uid 
      
        &&
           
              (    !("resource" in request)    ||    !("admin" in request.resource.data)    )
      
      )
      
      ||
     
      request.auth.uid == 'UID'
     
          
    }
    
  }
}