How to add a sprint button on a 3D platformer? In C#

I am making a 3D platformer and I can’t figure out how to add a sprint to this script in C#.
using UnityEngine;
using System.Collections;

public class Controller : MonoBehaviour {
	public float speed = 6.0F;
	public float sprint = 8.0F;
	public float jumpSpeed = 8.0F;
	public float gravity = 20.0F;
	private Vector3 moveDirection = Vector3.zero;
	void Update() {
		CharacterController controller = GetComponent<CharacterController>();
		if (controller.isGrounded) {
			moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
			moveDirection = transform.TransformDirection(moveDirection);
			moveDirection *= speed;
			if (Input.GetButton("Jump"))
				moveDirection.y = jumpSpeed;
			
		}
		moveDirection.y -= gravity * Time.deltaTime;
		controller.Move(moveDirection * Time.deltaTime);
	}
}

public class Controller : MonoBehaviour {
public float speed = 6.0F;
public float sprint = 8.0F;
public float jumpSpeed = 8.0F;
public float gravity = 20.0F;
private Vector3 moveDirection = Vector3.zero;
void Update() {
CharacterController controller = GetComponent();
if (controller.isGrounded) {
moveDirection = new Vector3(Input.GetAxis(“Horizontal”), 0, Input.GetAxis(“Vertical”));
moveDirection = transform.TransformDirection(moveDirection);
if (Input.GetKey (KeyCode.Shift))
moveDirection *= speed * 2;
else
moveDirection *= speed;
if (Input.GetButton(“Jump”))
moveDirection.y = jumpSpeed;

             }
             moveDirection.y -= gravity * Time.deltaTime;
             controller.Move(moveDirection * Time.deltaTime);
         }
     }