左边DIV固定宽度，右边的DIV如何自适应填满剩下的屏幕宽度

    <style>
        .main{
            margin-top: -20px;
            width: 100%;
            height: auto;
            overflow: hidden;
        }
        .main .left{
            float: left;
            width: 300px;
            height: 700px;
            border-right: 1px solid #DDDDDD;
        }
        .main .right{
            margin-left: 300px;
            height: 700px;
            background-color: red;
        }
    </style>