SOURCE

<div class="box">
    <div class='left'>
        <p>01</p>
        <p>02</p>
        <p>03</p>
        <p>04</p>
        <p>05</p>
        <p>06</p>
        <p>07</p>
        <p>08</p>
        <p>09</p>
        <p>10</p>
    </div>
    <div class="right">right</div>
</div>

 

.box {
    background: #ccc;
    display: flex;
}

.left {
    width: 300px;
    border: 1px solid #333;
}

.right {
    width: 200px;
    border: 1px solid #f00;
}