Add class to div when user scroll down
<p>i need to Add class to div when user scroll down, this is my code:</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>$(window).scroll(function() { 
   $("#skin_h").removeClass("skin");
   var scroll = $(window).scrollTop();
   if (scroll &lt;= 75) {
      $("#skin_h").addClass("skin_srolld");
   }
});</code></pre>
<pre class="snippet-code-css lang-css prettyprint-override"><code>.skin {
    position: fixed;
    z-index: 0;
    top: 0px;
    left: 50%;
}


.skin_srolld {
    top:500px!important;
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="skin" id="skin_h"&gt;
&lt;/div&gt;</code></pre>
</div>
</div>
</p>

<p>its not work some help with this please.</p>