title: what is n in nth:child selector in css
body: <p>I am curious about nth child selector in css. But is hard to understand its logic for me now. Below given example highlight first and the last span.</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;div&gt;
  &lt;span&gt;This span is limed!&lt;/span&gt;
  &lt;span&gt;This span is not. :(&lt;/span&gt;
  &lt;em&gt;This one is odd. &lt;/em&gt;
  &lt;span&gt;Sadly, this one is not...&lt;/span&gt;
  &lt;span&gt;But this one is!&lt;/span&gt;
  &lt;/div&gt;
  </code></pre>
  
  <pre class="lang-css prettyprint-override"><code>span:nth-child(2n+1) {
  background-color: green;
  }
  </code></pre>