Добавить автопрокрутку на слайдер

var slideIndex = 1; 
showSlides(slideIndex); 
 
function plusSlides(n) { 
  showSlides(slideIndex += n); 
} 
 
function currentSlide(n) { 
  showSlides(slideIndex = n); 
} 
 
function showSlides(n) { 
  var i; 
  var slides = document.getElementsByClassName('slide'); 
  var dots = document.getElementsByClassName('dot'); 
 
  if (n > slides.length) { 
    slideIndex = 1; 
  } 
  if (n < 1) { 
    slideIndex = slides.length; 
  } 
  for (i = 0; i < slides.length; i++) { 
    slides[i].style.display = "none"; 
 
  } 
  for (var i = 0; i < dots.length; i++) { 
    dots[i].className = dots[i].className.replace("active", ""); 
  } 
  slides[slideIndex - 1].style.display = "block"; 
  dots[slideIndex - 1].className += " active"; 
} 
var i = 1; 
  setInterval(function(){ 
  currentSlide(i) 
  i++; 
  if(i == 5) 
  { 
   i = 1; 
  } 
},2000)

<div id="slider"> 
  <div class="slide fade"> 
    <div class="number-text">1/4</div> 
    <img src="img/slide1.jpg" style="width: 100%" alt=""> 
    <div class="text">Первый слайд</div> 
  </div> 
  <div class="slide fade"> 
    <div class="number-text">2/4</div> 
    <img src="img/slide2.jpg" style="width: 100%" alt=""> 
    <div class="text">Второй слайд</div> 
  </div> 
  <div class="slide fade"> 
    <div class="number-text">3/4</div> 
    <img src="img/slide3.jpg" style="width: 100%" alt=""> 
    <div class="text">Третий слайд</div> 
  </div> 
  <div class="slide fade"> 
    <div class="number-text">4/4</div> 
    <img src="img/slide4.jpg" style="width: 100%" alt=""> 
    <div class="text">Четвертый слайд</div> 
  </div> 
  <a class="prev" onclick="plusSlides(-1)"> 
    <img src="img/arrow-left.png" alt=""> 
  </a> 
  <a class="next" onclick="plusSlides(1)"> 
    <img src="img/arrow-right.png" alt=""> 
  </a> 
</div> 
<br> 
<div style="text-align: center;"> 
  <span class="dot active" onclick="currentSlide(1)"></span> 
  <span class="dot" onclick="currentSlide(2)"></span> 
  <span class="dot" onclick="currentSlide(3)"></span> 
  <span class="dot" onclick="currentSlide(4)"></span> 
</div>