FPS movements.

Hi, i need some help! I searched in this site a script to add some movements (crouch and run) on FPS controller: here is the script

  var walkSpeed: float = 7; // regular speed
  var crchSpeed: float = 3; // crouching speed
  var runSpeed: float = 20; // run speed
  
  private var chMotor: CharacterMotor;
  private var ch: CharacterController;
  private var tr: Transform;
  private var height: float; // initial height
  
  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")){
      speed = runSpeed;
      
  }
  if (Input.GetKey("left ctrl")){ // press Ctrl to crouch
      h = 0.5 * height;
      speed = crchSpeed; // slow down when crouching
  }
  chMotor.movement.maxForwardSpeed = speed; // set max speed
  var lastHeight = ch.height; // crouch/stand up smoothly 
  ch.height = Mathf.Lerp(ch.height, h, 5*Time.deltaTime);
  tr.position.y += (ch.height-lastHeight)/2; // fix vertical position
  }

If I press LeftShift to run and then I release it, the character slides on the floor too, but I want avoid this.

How can I adjust this script?

P.S: Sorry for my bad english and my ignorance.