E se io avessi degli importi (input text) non soggetti a moltiplicatore, da sommare a quelli soggetti a moltiplicatore come cambierebbe la funzione per fare il totale?

codice:
<html>

	<head>

<script language="javascript" type="text/javascript">


var campi = new Array("imp1", "imp2", "imp3"); 

function somma(ff) {
  var tot = 0;
  for(var i=0; i<campi.length; i++) {
    var c = ff.elements[campi[i]].value;
    if(c=="") c="0";
    if(c=='NaN') {alert("Not A Number"); c=0;}
    tot += parseInt(c,10);
  }
  ff.Totale.value = tot * ff.MoltImp1.options[ff.MoltImp1.selectedIndex].value;
}

</script>

	</head>

			<body>	

			<form name="modulo">	

			<table align="center">


			<tr>
			<td>Campo 1</td>
			<td><input name="campo1" type="text" size="10" onblur="somma(this.form)" />  </td>
			<td></td>
			</tr>

			<tr>
			<td>Campo 2</td>
			<td><input name="campo2" type="text" size="10" onblur="somma(this.form)" />  </td>
			<td></td>
			</tr>

			<tr>	

			<td>Importo 1</td>
			<td><input name="imp1" type="text" size="10" onblur="somma(this.form)" />  </td>
			
			<td><select name="MoltImp1" size="1" maxLength="50">
  			
           		<option value="1">1</option>
           		<option value="2">2</option>
           		<option value="4">3</option>
           		<option value="3">4</option>
           		<option value="6">6</option>
           		<option value="12" selected>12</option>
			</select>			

			</td>
			</tr>

			<tr>
			<td>Importo 2</td>
			<td><input name="imp2" type="text" size="10" onblur="somma(this.form)" />  </td>
			
			<td><select name="MoltImp2" size="1" maxLength="50">
  			
           		<option value="1" selected>1</option>
           		<option value="2">2</option>
           		<option value="4">3</option>
           		<option value="3">4</option>
           		<option value="6">6</option>
           		<option value="12">12</option>
			</select>


			</td>
			</tr>
		
			<tr>
			<td>Importo 3</td>
			<td><input name="imp3" type="text" size="10" onblur="somma(this.form)" /> </td>
			
			<td><select name="MoltImp3" size="1" maxLength="50">
  			
           		<option value="1" selected>1</option>
           		<option value="2">2</option>
           		<option value="4">3</option>
           		<option value="3">4</option>
           		<option value="6">6</option>
           		<option value="12">12</option>
			</select>
	
			</td>
			</tr>

			<tr>
			<td>TOTALE</td>
			<td><input name="Totale" type="text" size="10" readonly="readonly" /></td>
			</tr>

			</table>

			</body>

			</form>

</html>