为什么我这输出是NAN？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>定义函数</title>
<script type="text/javascript">
function sub2(a,b)
{
var aaa = a-b;
document.getElementById("sss").innerHTML+=aaa;
}
sub2(3,2);
sub2(4,2);
sub2(5,2);
</script>
</head>
<body>
<input type="button" value="点击我" onclick="sub2()" />
<div id="sss"></div>
</body>
</html>
