How to Stop Camera From Rotating when x,y,and z is equal to 90 degrees

Here is my code i need to stop the camera from rotating when its 90 degrees

  public float borderthickness = 10f;
     public float cameramovespeed = 10000f;
     
 
     // Update is called once per frame
     void Update()
     {
         Quaternion pos = transform.rotation;
 
   
             if (Input.mousePosition.y >= Screen.height - borderthickness && pos.x < 90f)
             {
                 pos.x -= cameramovespeed * Time.deltaTime;
                Debug.Log("up");
 
             }
            if (Input.mousePosition.y <= borderthickness && pos.x < 90f)
           {
             pos.x += cameramovespeed * Time.deltaTime;
    Debug.Log("down");
             
         }
         if (Input.mousePosition.x >= Screen.width - borderthickness && pos.y < 90f)
         {
             pos.y += cameramovespeed * Time.deltaTime;
             
    Debug.Log("right");
         }
         if (Input.mousePosition.x <= borderthickness && pos.y < 90)
         {
             pos.y -= cameramovespeed * Time.deltaTime;
                Debug.Log("left");
 
         }
         transform.rotation = pos;