Bootstrap 3 Clear Rows

HTML

<link rel="stylesheet" href="http://getbootstrap.com/dist/css/bootstrap.css">
<script src="http://getbootstrap.com/dist/js/bootstrap.js"></script>
<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/font-awesome/4.1.0/css/font-awesome.min.css">
<div class="container-fluid">
    <div class="row-fluid">
        <div class="col-xs-6">
            1
        </div>
        <div class="col-xs-6">
           2
        </div>
        <div class="col-xs-6 label-warning">
           3
        </div>
        <div class="col-xs-6">
          4<br/>
          s<br/>
          s<br/>
          s<br/>
          s<br/>
          s
        </div>
        <div class="col-xs-6 label-success">
          5
        </div>
        <div class="col-xs-6">
          6
        </div>
    </div>
</div>

CSS

.col-xs-6:nth-child(even) {
    clear: right;
}
.col-xs-6 {
    border: 1px solid grey;
}