what this bug

this is code :

using UnityEngine;

using UnityEngine.UI;

using Photon.Pun;

public class NetworkManager : Photon.PunBehaviour {

    public GameObject lobbyCam;

    public Transform spawnPoint;

    public Text statusText;

    public const string Version = "1.0";

    public const string RoomName = "Multiplayer";

    public string playerPrefabName = "Player";


this is error message :

error CS0234: The type or namespace name 'PunBehaviour' does not exist in the namespace 'Photon' (are you missing an assembly reference?)