我的这个程序怎么不动的，哪里错了啊
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>jQuery动画特效</title>
<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js" type="text/javascript"></script>
</head>
<style>
div{
top:50px;
position:absolute;
width:80px;
height:80px;
background:red;
left:0px;}
</style>
<body>
<div ></div>
<input id="left" type="button" value="左移"/>
<input id="right" type="button" value="右移"/>
<span></span>
</body>
<script type="text/javascript">
$ (function(){
$("#left").bind("click",function(){
$("div").animate({
left: '+=50px'
},300,function(){$("span").html("左移成功");});
$("#right").bind("click",function(){
$("div").animate({
left: '-=50px'
},300,function(){$("span").val("右移成功");});
})
</script>
</html>
