fps movment unity

public float walkSpeed = 6.0F;
public float jumpSpeed = 8.0F;
public float runSpeed = 8.0F;
public float gravity = 20.0F;

private Vector3 moveDirection = Vector3.zero;
private CharacterController controller;

void Start()
{
    controller = GetComponent<CharacterController>();
}

void Update()
{
    if (controller.isGrounded)
    {
        moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
        moveDirection = transform.TransformDirection(moveDirection);
        moveDirection *= walkSpeed;
        if (Input.GetButton("Jump"))
            moveDirection.y = jumpSpeed;
    }
    moveDirection.y -= gravity * Time.deltaTime;
    controller.Move(moveDirection * Time.deltaTime);
}