How to count the number of pages of owl corusel 2 slides?

Question

Owl Carousel

var owl = $('.b-sale__action');
owl.owlCarousel({
  items : 4,
  slideBy: 4,
  loop: true,
  onInitialized: function(e) {
    $('.counter-out').text('1 / ' + this.items().length)
    console.log();
  }
});
owl.on('changed.owl.carousel', function(e) {
  $('.counter-out').text(++e.page.index  + ' / ' + e.page.count)
});

I'm using a script to count slides for owl corusel 2. But here's the problem: I can't display the number of slide screens (number of groups of 4 slides) into this.items().length variable. How to count the number of slide groups and not all slides?