Pause Game

I have a script from other forums for first person movement. I want to stop the movement when you press escape. I have no idea how to this, any help is appreciated, thanks!

 using UnityEngine;
 using System.Collections;
     
     [RequireComponent (typeof(CharacterController))]
     public class FirstPersonController : MonoBehaviour {
         
         public float movementSpeed = 5.0f;
         public float mouseSensitivity = 5.0f;
         public float jumpSpeed = 20.0f;
         
         float verticalRotation = 0;
         public float upDownRange = 60.0f;
         
         float verticalVelocity = 0;
         
         CharacterController characterController;
         
         // Use this for initialization
         void Start () {
             Screen.lockCursor = true;
             characterController = GetComponent<CharacterController>();
         }
         
         // Update is called once per frame
         void Update () {
             // Rotation
             float rotLeftRight = Input.GetAxis("Mouse X") * mouseSensitivity;
             transform.Rotate(0, rotLeftRight, 0);
     
             
             verticalRotation -= Input.GetAxis("Mouse Y") * mouseSensitivity;
             verticalRotation = Mathf.Clamp(verticalRotation, -upDownRange, upDownRange);
             Camera.main.transform.localRotation = Quaternion.Euler(verticalRotation, 0, 0);
             
     
             // Movement
             
             float forwardSpeed = Input.GetAxis("Vertical") * movementSpeed;
             float sideSpeed = Input.GetAxis("Horizontal") * movementSpeed;
             
             verticalVelocity += Physics.gravity.y * Time.deltaTime;
             
             if( characterController.isGrounded && Input.GetButton("Jump") ) {
                 verticalVelocity = jumpSpeed;
             }
             
             Vector3 speed = new Vector3( sideSpeed, verticalVelocity, forwardSpeed );
             
             speed = transform.rotation * speed;
             
             
             characterController.Move( speed * Time.deltaTime );
         }
     }