@IBAction func insertAndDeleteButtonTapped(_ sender: Any) {
        collectionView.performBatchUpdates({
            print(collectionView.numberOfItems(inSection: 0))
            let index = 1
            for i in (index..<numbers.count).reversed() {
                numbers.remove(at: i)
                collectionView.deleteItems(at: [IndexPath(item: i, section: 0)])
                print(collectionView.numberOfItems(inSection: 0))
            }
            numbers.insert(100, at: index)
            collectionView.insertItems(at: [IndexPath(item: index, section: 0)])
            print(collectionView.numberOfItems(inSection: 0))
        }, completion: { _ in
            print(self.collectionView.numberOfItems(inSection: 0))
        })
    }


//output
3
3
3
3
2