enter an int
 

<html>
<head>
<script language ="JavaScript">
<!--
function calc(num)
{var i,sum=0
for(i=1;i<=num;i++)sum+=i
return sum

}
//-->
</script>
</head>
<body>
<form name="form1">
enter an int
<input type="text" name="text2">
<input type="button" value="calculate" onclick="alert(calc(text2.value))"><br>

</form>
</body>
</html>