Get Category of the Ad from CategoryItems - Laravel 5.4

Could you please help me to get a Category name of the specific Ad (Post) from the CategoryItems table.

Ad Model:

public function cat()
{
  return $this->belongsTo(Category::class);
}

public function categoryItems()
{
    return $this->hasMany(CategoryItems::class);
}

Category Model:

public function ads()
{
  return $this->hasMany(Ad::class);
}

public function categoryItems()
{
  return $this->hasMany(CategoryItems::class);
}

CategoryItems Model:

public function cat()
{
  return $this->belongsTo(Category::class);
}

public function ads()
{
  return $this->hasMany(Ad::class);
}

AdsController Controller:

public function show($slug)
{
  $ad = Ad::where('slug', $slug)->firstOrFail();
  $cat = Category::get();

  return view('ads.show', compact('ad', 'cat'));
}

View:

<h2></h2>
<div class="price">
  Price: 
</div>
<div class="city">
  <h3></h3>
</div>
<div class="cat">
  <h3></h3>
</div>
<p></p>
<p>Posted at: </p>

What I get now in the "cat" block is:

[{"id":2,"category_id":5,"ad_id":4,"created_at":"2017-07-18 10:37:33","updated_at":"2017-07-18 10:37:33"}]

I don't get it any further...

Update:

Ads DB:

id | title | slug | body | price | city | created_at | updated_at

Categories DB:

id | title | slug | created_at | updated_at

CategoryItems DB:

id | category_id | ad_id | created_at | updated_at

via Chebli Mohamed