Caot add image in HTML + CSS Slider

I am trying to customize the slider given here by adding one more image in the HTML.

When i add an image in HTML and make respective changes in the CSS the code is breaking and its not working.

Here is the HTML code of the slider:

<base href="https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/">
<div id="slider">
<figure>
<img src="austin-fireworks.jpg" alt>
<img src="taj-mahal_copy.jpg" alt>
<img src="ibiza.jpg" alt>
<img src="ankor-wat.jpg" alt>
<img src="austin-fireworks.jpg" alt>
</figure>
</div>

Here is the CSS code:

@keyframes slidy {
0% { left: 0%; }
20% { left: 0%; }
25% { left: -100%; }
45% { left: -100%; }
50% { left: -200%; }
70% { left: -200%; }
75% { left: -300%; }
95% { left: -300%; }
100% { left: -400%; }
}

body { margin: 0; } 
div#slider { overflow: hidden; }
div#slider figure img { width: 20%; float: left; }
div#slider figure { 
  position: relative;
  width: 500%;
  margin: 0;
  left: 0;
  text-align: left;
  font-size: 0;
  animation: 30s slidy infinite; 
}

I tried adding the image inside the <figure> and the slider stops working.

You can run the code at the Codepen.io website.