Can someone help me add running to my script?

Hi everyone I am stumped on this running bit. Im trying to add running to my script but I cant seem to make it work. Im trying to run with the left and right shift key. Please help here is the script I have for movement:

public var MoveSpeed :float = 10;
	
	public var jumpSpeed : float = 0;
	
		public var MoveDirection = Vector3.zero;
		
			public var Gravity :float = 45;
			
				public var grounded : boolean = false;

function Update () {

if(grounded){

	MoveDirection = new Vector3(Input.GetAxis("Horizontal"), 0 , Input.GetAxis("Vertical"));
	
		MoveDirection = transform.TransformDirection(MoveDirection);
		
			MoveDirection *= MoveSpeed;

}

	if(grounded){
	
		if(Input.GetKey(KeyCode.Space)){
		
			MoveDirection.y = jumpSpeed;

}

}

MoveDirection.y -= Gravity * Time.deltaTime;

			var Controller = GetComponent(CharacterController);
			
				var Flags = Controller.Move(MoveDirection * Time.deltaTime);
				
					grounded = (Flags & CollisionFlags.CollidedBelow) !=0;

}

I would use Input.GetKeyDown and modify your MoveSpeed variable (i.e. MoveSpeed *= 1.5f).

Then on Input.GetKeyUp, I would set MoveSpeed back to default