为什么输出的值为NaN？
<html>
<head>
<script type="text/javascript">
function add2(){
var numa=prompt("a").value;
var numb=prompt("b").value;
var sum=numa+numb;
document.getElementById("sss").innerHTML=sum;
}
</script>
</head>
<body>
<input name="button" type="button" value="点击提交" onclick="add2()" />
<div id="sss"></div>
</body>
</html>
