i have the following simple script

<div id="content">
   <div id="left">
    <div id="menu">
     <ul>
      <li>menu</li>
     </ul>
    </div>
    <div id="left_ad">
    </div>
   </div>
   <div id="middle">
    some text here...
   </div>
   <div id="right">
    <div id="right_ad">
     <div id="ad2">
     </div>
    </div>
   </div>
  </div>

css:

#content
{

 margin: 0;
 padding: 0;
 overflow: hidden;
 position: relative;

}
#left
{
 float: left;
 width:25%;
 margin-left:0;
 margin-right:0;
 margin-top:0;
 margin-bottom:-5000px;
 padding-left:0;
 padding-right:0;
 padding-top:0;
 padding-bottom:5000px;

}
#middle
{
 background-image:url("http://localhost/kino/img/theme.png");
 background-repeat:no-repeat;
 float: left;
 width:50%;
 margin-left:0;
 margin-right:0;
 margin-top:0;
 margin-bottom:-5000px;
 padding-left:0;
 padding-right:0;
 padding-top:0;
 padding-bottom:5000px;
}
#right
{
 float: left;
 width:25%;
 margin-left:0;
 margin-right:-1px;
 margin-top:0;
 margin-bottom:-5000px;
 padding-left:0;
 padding-right:0;
 padding-top:0;
 padding-bottom:5000px;
}
#left_ad
{
 background-image:url("http://localhost/kino/img/lens1.png");
 background-repeat:no-repeat;
 min-height:498px;
 margin-left: auto;
 margin-right: auto;
 margin-top:31px;
 padding:0;
 width: 188px;
 position: relative;
 height: 100%;
}

as you see, left, middle and right have the same size of heigth, which is equal to max of them. and now, what is the question, i want left_ad to have the same heigth too. how can i set the heigth of left_id is equal to left? thanks