Jump not working

Hello. I writed this code by following brackeys video but i don't know why the character don't jump. Please help me. (3D game) Here is the code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerMovement : MonoBehaviour
 {
     public CharacterController controller;
 
     public float speed = 12f;
     public float gravity = -19.62f;
     public float jumpHeight = 3.88f;
 
     public Transform groundCheck;
     public float groundDistance = 0.4f;
     public LayerMask groundMask;
     
     Vector3 velocity;
     bool isGrounded;
 
     // Update is called once per frame
     void Update()
     {
       isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
 
       if(isGrounded && velocity.y < 0);
       {
         velocity.y = -2f;
       }
 
       float x = Input.GetAxis("Horizontal");
       float z = Input.GetAxis("Vertical");
 
       Vector3 move = transform.right * x + transform.forward * z;
       
       controller.Move(move * speed * Time.deltaTime);
 
       if(Input.GetButtonDown("Jump") && isGrounded)
       {
         velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
       }
     
       velocity.y += gravity * Time.deltaTime;
       controller.Move(velocity * Time.deltaTime);
     }
 }