Count order with certain product

Question

I need count the orders of a given customer only if the order contains a specific category, how to modify this to get what I want ?

ex : if the order has a product from the category "watches", I want to count in the result the this query.

EDIT

Basically I need this :

    $nbOrders = 0;
        foreach($orders as $order){
         $items = $order->getAllItems();
         foreach($items as $item){
          $id = $item->getProductId();
          $categories = $item->getCategory();
          if(in_array("specific category", $categories)) $nbOrders++;
        }

}

But with factory it would be easier and cleaner

protected $_orderCollectionFactory;
protected $orders;
public function __construct(
    .....
    \Magento\Sales\Model\ResourceModel\Order\CollectionFactory $orderCollectionFactory,
    .......
) {
    ...
    $this->_orderCollectionFactory = $orderCollectionFactory;
    ........
    parent::__construct($context, $data);
}


public function getOrders()
{

    if (!$this->orders) {
        $this->orders = $this->_orderCollectionFactory->create()->addFieldToSelect('*')->addFieldToFilter('customer_id',$customerId)->setOrder('created_at','desc');
    }
    return $this->orders;
}