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<div class="container">
    <div class="row">
        <div class="col-sm-6">
            <div class="col-sm-6">1</div>
            <div class="col-sm-6">2</div>
        </div>
        <div class="col-sm-6">
            3
        </div>
    </div>
    <div class="row">
        <div class="col-sm-6">
            <div class="row">
                <div class="col-sm-6">4</div>
                <div class="col-sm-6">5</div>
            </div>
        </div>
        <div class="col-sm-6">
            6
        </div>
    </div>
</div>

/* Latest compiled and minified CSS included as External Resource*/

/* Optional theme */
@import url('//netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap-theme.min.css');

body {
    margin: 10px;
}

.col-sm-6 > .col-sm-6 {
    background-color: cyan;
}

.col-sm-6 > .row > .col-sm-6 {
    background-color: lightgray;
}

/* Latest compiled and minified JavaScript included as External Resource */