How do I add a sprint recharge

I have a basic sprint script that allows the player to walk using "W" run using "Shift" and Crouch using "c". As the player sprints the stamina goes down. And it stays down. I want it to go back up to 100 so the player can sprint again. here is the script

 var walkSpeed: float = 8;
 var crchSpeed: float = 3;
 var runSpeed: float = 16;
 var stamina : int = 100;
  
 private var chMotor: CharacterMotor;
 private var ch: CharacterController;
 private var tr: Transform;
 private var height: float;
  
 function Start() {
     chMotor = GetComponent(CharacterMotor);
     tr = transform;
     ch = GetComponent(CharacterController);
     height = ch.height;
 }
  
 function Update() {
     var h = height;
     var speed = walkSpeed;
     if (ch.isGrounded && Input.GetKey("left shift") || Input.GetKey("right shift")) {
         if (stamina != 0) {
             speed = runSpeed;
             stamina = stamina - 1;
         }
     }
     if (Input.GetKey("c") || Input.GetKey("c")) {
         h = 0.5 * height;
         speed = crchSpeed;
     }
     chMotor.movement.maxForwardSpeed = speed;
     var lastHeight = ch.height;
     ch.height = Mathf.Lerp(ch.height, h, 5*Time.deltaTime);
     tr.position.y += (ch.height-lastHeight)/2;
  
     if (stamina != 100) {
             stamina = stamina + 0.5;
         }
 }
  
 function OnGUI () {
     GUI.Box(Rect(120,Screen.height - 50, 80, 40), "");
     GUI.Label(Rect(130,Screen.height - 30, 180, 125), stamina + " / " + "100");
     GUI.Label(Rect(130,Screen.height - 50, 180, 125), "Stamina");
 }