AddForce Movement Constraint

I was experimenting with the addforce command and how to make an object move with it. I made a script with it and found that whenever I spam the jump key that the object keeps on moving upwards.

     public Rigidbody rB;

 public float velocityF = 200f;


 void Start()
 {

 }
 // Update is called once per frame
 void Update () 
 {
     if (Input.GetKeyDown ("d")) 
     { 
         rB.AddForce (velocityF, 0, 0 * Time.deltaTime);
     }    
     if (Input.GetKeyDown ("a")) 
     {
         rB.AddForce (-velocityF, 0, 0 * Time.deltaTime);
     }    
     if (Input.GetKeyDown ("s")) 
     {
         rB.AddForce (0,-velocityF , 0 * Time.deltaTime);
     }
     if (Input.GetKeyDown ("w")) 
     {
         rB.AddForce (0,velocityF , 0 * Time.deltaTime);
     }
 }

How can I make the object jump only two times before landing?