How do i code double jumps to character controller on unity?

heres my script. i just started coding like 3 days ago so idk much.

 public class playermovement : MonoBehaviour {    Vector3 velocity;     public float gravity = -9.81f;     public CharacterController controller;     public Transform groundCheck;     public float groundDistance = 0f;     public LayerMask groundMask;     bool isGrounded;     public float jumpHeight = 3f;             // Update is called once per frame     void Update()     {         velocity.y += gravity * Time.deltaTime;         controller.Move(velocity * Time.deltaTime);          isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);         if (isGrounded && velocity.y < 0)         {              velocity.y = -2f;         }         if (Input.GetButtonDown("Jump") && isGrounded)         {             velocity.y = Mathf.Sqrt(jumpHeight * -2 * gravity);          }                } }