I need character only jump

I have a character with this script:

 var speed : float = 6.0;
     var jumpSpeed : float = 8.0;
     var gravity : float = 20.0;
 
     private var moveDirection : Vector3 = Vector3.zero;
 
     function Update() {
         var controller : CharacterController = GetComponent(CharacterController);
         if (controller.isGrounded) {
             // We are grounded, so recalculate
             // move direction directly from axes
             moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
                                     Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             
             if (Input.GetButton ("Jump")) {
                 moveDirection.y = jumpSpeed;
             }
         }
 
         // Apply gravity
         moveDirection.y -= gravity * Time.deltaTime;
         
         // Move the controller
         controller.Move(moveDirection * Time.deltaTime);
     }

With this code, character is moving to left and right. I need that only jump.

How to make it?