I can't get variable to decrease by one

This is probably a very easy question to answer, but for some reason, I can't figure out what is wrong. I want my int showAd to decrease by one every time the Canvas1 is enabled. Here is code: using System.Collections; using System.Collections.Generic; using UnityEngine; using GoogleMobileAds.Api;

 public class AdManager : MonoBehaviour {
 
     //public bool dead;
     public int showAd = 3;
     public InterstitialAd interstitial;
     public BannerView bannerView;
     public bool canShowInt;
 
     void Start () {
         RequestBanner ();
         RequestInterstitial ();
     }
 
     void Awake () {
         DontDestroyOnLoad (this);
     }
 
     void Update () {
         if (GameObject.Find ("Canvas1").GetComponent <Canvas> ().enabled == true) {
             canShowInt = true;
         } else {
             canShowInt = false;
         }
         if (showAd == 0) {
             showAd = 3;
         }
     }
 
 
     void ShowInterstitial () {
         if (canShowInt == true) {
             showAd = showAd - 1;
             if (showAd == 0) {
                 if (interstitial.IsLoaded ()) {
                     interstitial.Show ();
                     interstitial.Destroy ();
                 }
             }
         }
     }