Cube Movement using Touch??

cube is work fine left,right,up,down fine

problem is

cube up and down : but when i m comment a code left and right then work up and down cube left and right : but when i m comment a code up and down then work left and right

     Rigidbody2D rb;
     public float movespeed;
 
     // Use this for initialization
     void Start()
     {
         rb = GetComponent<Rigidbody2D>();
         movespeed = 10f;
 
         Debug.Log("Screen Width : " + Screen.width);     //1080
         Debug.Log("Screen Height : " + Screen.height);   //1920
 
     }
 
     // Update is called once per frame
     void Update()
     {
         if (Input.touchCount > 0)
         {
             Touch touch = Input.GetTouch(0);   ///hold the information about this touch
 
             switch (touch.phase)
             {
                 case TouchPhase.Began:

                      // problem is left and right work but when  i comment a code up and down
                     if (touch.position.x < Screen.width / 2)     //left
                         rb.velocity = new Vector2(-movespeed, 0f);
                     DebugPanel.Log("x", "first W:", touch.position.x < Screen.width / 2);
 
                     if (touch.position.x > Screen.width / 2)  //right
                         rb.velocity = new Vector2(movespeed, 0f);
                     DebugPanel.Log("x", "Second W:", touch.position.x > Screen.width / 2);


                    // problem is up and down work but when  i comment a code left and right
                     if (touch.position.y < Screen.height / 2)    //Up
                     rb.velocity = new Vector2(0f, -movespeed);
                     DebugPanel.Log("y", "First H :", touch.position.y < Screen.height / 2);
 
                     if (touch.position.y > Screen.height / 2)  //Down
                         rb.velocity = new Vector2(0f, movespeed);
                     DebugPanel.Log("y", "Second H:", touch.position.y > Screen.height / 2);
 
                     break;
 
                 case TouchPhase.Ended:
                     rb.velocity = new Vector2(0f, 0f);      //when cube is ended then velocty is0
                     break;
             }
         }
     }

how to move my cube when i touch left then going left, when i touch right then cube going right, when i touch upthen cube going up , when i touch down then cube going down.. hOW??

cube is going out of screen in mobile??