Buongiorno raga,
ho questa form html

codice:
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Documento senza titolo</title>


<script language="javascript">
function sum() {
var a = parseInt(document.all.imp_1.value);
var b = parseInt(document.all.iva_1.value);
var c = a + b;
document.all.tot_1.value = c;
}
</script>


</head>


<body>
<legend>Interventi</legend>
<table width="615" border="1" cellpadding="3" cellspacing="1" bordercolor="#FFCC00" style="background-color:#FFFFCC">
        <td>Intervento</td>
      	<td>Importo</td>
        <td>Iva</td>
        <td>Calcola</td>
        <td>Totale</td>
        </tr>
      <tr>
        </tr>
      <tr>
        <td><input name="int1" id="int1" size="40"></td>
        <td><input name="imp_1" id="imp_1" size="10"></td>
        <td><input name="iva_1" id="iva_1" size="10"></td>
        <td><input type="button" value="Calcola" onClick="javascript:sum();"></td>
        <td><input name="tot_1" id="tot_1" size="12"></td>
        </tr>
        </table>
</fieldset>
</body>
</html>
vorrei adattare lo script javascript per calcolare l'iva, mi dareste una mano ?

Grazie