Can anyone help me out with counter movement?

How do I add counter movement to this script? thanks ;)

public float moveSpeed = 300; public GameObject character;

 private Rigidbody2D characterBody;
 private float ScreenWidth;


 // Use this for initialization
 void Start()
 {
     ScreenWidth = Screen.width;
     characterBody = character.GetComponent<Rigidbody2D>();
 }

 // Update is called once per frame
 void Update()
 {
     int i = 0;
     //loop over every touch found
     while (i < Input.touchCount)
     {
         if (Input.GetTouch(i).position.x > ScreenWidth / 2)
         {
             //move right
             RunCharacter(1.0f);
         }
         if (Input.GetTouch(i).position.x < ScreenWidth / 2)
         {
             //move left
             RunCharacter(-1.0f);
         }
         ++i;
     }
 }
 void FixedUpdate()
 {

     RunCharacter(Input.GetAxis("Horizontal"));

 }

 private void RunCharacter(float horizontalInput)
 {
     //move player
     characterBody.AddForce(new Vector2(horizontalInput * moveSpeed * Time.deltaTime, 0));

 }