click button and enter data as prompted
 

<html>
<head>
<script language ="JavaScript">
<!--
var num1,num2
function getInts()
{
num1=prompt("enter first int:",1)
num2=prompt("enter second int:",2)
}
function calc()
{
var i,start,last,sum=0
start=parseInt(num1)
last=parseInt(num2)
for(i=start;i<=last;i++)
   {sum=sum+i}
alert("sum of ints "+num1+ "to "+num2+ "is\n\n\t "+sum)
}

//-->
</script>
</head>
<body>
<form>
click button and enter data as prompted
<input type="button" value="calculate"
onclick="getInts();calc()"><br>

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