Can't it be solved with a javascript function? it isn't that complicated and it frees you from the need to support the languages such as php or c#.
EDIT:
<html>
<head>
<script>
function add()
{
document.getElementById("sum").value = Number(document.getElementById("num1").value) + Number(document.getElementById("num2").value);
}
</script>
</head>
<body>
<body>
<body bgcolor="#DDA0DD">
<h1>"CALCULATION"</h1>
1st Number:<input type="text" id="num1"/>
<br>
2nd Number:<input type="text" id="num2"/>
<br>
Sum: <input type="text" id="sum"/>
<br>
<button onclick="add()">CLICK ME NOW</button>
<br>
<h1><B><font color="BLUE">GETTING THERE SLOWLY BUT SURELY BABY!!! :)</B></h1>
</form>
<p></p>
</body>
</html>
this code should do the trick
Note: I have not tested it so give it a try and let me know how well it runs.