移动端absolute元素居中

<div class="db">
    <div class="dm"></div>
</div>

.db{
    400px;
    height:200px;
    background:#ccc;
}
.dm{
    50px;
    height:50px;
    background:pink;
    position:absolute;
    left:0;right:0;
    top:0;bottom:0;
    margin:auto;
}