为什么运行不了？这个错在哪了，求大神指点！
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>jQuery动画特效</title>
<style>
#div1{width:100px;height:100px;background:red;left:100px;position:relative;}
</style>
<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js" type="text/javascript"></script>
</head>
<body>
<input type="button" value="左移" id="btn1">
<input type="button" value="右移" id="btn2">
<div id="div1"></div>
<script>
$(function(){
$("#btn1").bind("click",function(){
$("div1").animate({left:"-=50px"},1000);
})
$("#btn2").bind("click",function(){
$("div1").animate({left:'+=50px'},1000);
})
})
</script>
</body>
</html>
