Code below is not working...nothing happens on clicking submit...javascript is enabled in browser..help please.

Code:
<html>

<head>

<script type="text/javascript">

function match()
{
var aa=document.getElementById("username").Value;

var bb=document.getElementById("password").Value;

if(aa==null || bb==null)
{
alert("Enter Again");
}


else if((aa=="admin") && (bb=="12345"))
{
Alert("Hello Sir");
}
else if((aa=="guest") && (bb=="67890"))
{
Alert("Hello Guest");
}
else
{
Alert("Enter Valid Info");
}



}
</script>



</head>

<body>


Username:<input type="text" id="username"/><br/>

Password:<input type="text" id="password"/><br/>

<input type="submit" onClick="match()"/>




</body>


</html>