Assuming by script you mean Javascript:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=windows-1250">
<meta name="generator" content="PSPad editor, www.pspad.com">
<title></title>
<SCRIPT language = JavaScript>
function calculate() {
A = eval(no1.value)
B = eval(no2.value)
C = (A + B)
result.value = C
}
</SCRIPT>
</head>
<body>
First Number : <input type="text" name="no1" /><br />
Second Number: <input type="text" name="no2" /><br />
Resulting num: <input type="text" name="result" /><br />
<Input Type = Button name = b1 value = "Add Numbers" onClick = calculate()>
</body>
</html>