client side calculation

Question

I want to calculate like below but its calling on any key press and calculation repeating,how to put condiot and where (where) when should clear variables values then it should'nt repeat..


<script type="text/javascript"> 

function sum() {
var packPrice = document.getElementById('txtAmntToPay').value;
var noOfCon = document.getElementById('txtNoOfConn').value;
var chargPerCon = document.getElementById('txtChargePerConn').value;
var installCharge = document.getElementById('txtInstallCharge').value;
var discount = document.getElementById('txtDiscount').value;
var final;


if (isNaN(packPrice)) {
packPrice = 0;
}
if (isNaN(noOfCon)) {
noOfCon = 0;
}
if (isNaN(chargPerCon)) {
chargPerCon = 0;
}
if (isNaN(installCharge)) {
installCharge = 0;
}
if (isNaN(discount)) {
discount = 0;
}
// final = parseFloat(packPrice);
var tot = parseInt(noOfCon) * parseInt(chargPerCon);
final = parseInt(packPrice) + parseInt(tot);
final = parseInt(final) + parseInt(installCharge);
final = parseInt(final) - parseInt(discount);
if (!isNaN(final)) {
document.getElementById('txtAmntToPay').value = final;
}
}
</script>


<table width="80%">
<tr>
<td width="20%">
Package Name:<br />
</td>
<td width="20%">
<asp:DropDownList ID="ddlPackName" runat="server" AutoPostBack="True" 
onselectedindexchanged="ddlPackName_SelectedIndexChanged" TabIndex="11">
<asp:ListItem Value="0">Select</asp:ListItem>
<asp:ListItem Value="200">Monthly</asp:ListItem>
<asp:ListItem Value="500">Quarterly</asp:ListItem>
<asp:ListItem Value="1000">HalfYearly</asp:ListItem>
<asp:ListItem Value="2000">Yearly</asp:ListItem>
</asp:DropDownList>

</td>
</tr>
</table>
<table width="80%">
<tr>
<td width="20%">
No. of Extra Connection:<br />
</td>
<td width="20%">
<asp:TextBox ID="txtNoOfConn" runat="server" onKeyup="sum();">0</asp:TextBox>

</td>
</tr>
</table>



<br />
<table width="80%">
<tr>
<td width="20%">
Charges per Connection(If any):<br />
</td>
<td width="20%">
<asp:TextBox ID="txtChargePerConn" runat="server" onKeyup="sum();">0</asp:TextBox>

</td>
</tr>
</table>
<br />
<table width="80%">
<tr>
<td width="20%">
Installation Charges:<br />
</td>
<td width="20%">
<asp:TextBox ID="txtInstallCharge" runat="server" onKeyup="sum();">0</asp:TextBox>

</td>
</tr>
</table>
<br />
<table width="80%">
<tr>
<td width="20%">
Discount (if any):<br />
</td>
<td width="20%">
<asp:TextBox ID="txtDiscount" runat="server" onKeyup="sum();">0</asp:TextBox>

</td>
</tr>
</table>
<br />
<table width="80%">
<tr>
<td width="20%">
Amount to Pay:<br />
</td>
<td width="20%">
<asp:TextBox ID="txtAmntToPay" runat="server" ReadOnly="True">0</asp:TextBox>
</td>
</tr>
</table>