Web Forms :: How To Add Textbox Values In FormView (Insert)

In FormView (insert) I have a number of textboxes into which number will be inputted. I need all these textboxes to add up and display the total in a Label which I can then insert into the database with all the individual values. I expected this to be a simple thing... Then it appears that the best way to do this is to use javascript, which I have never used! SO this is my attempt which is a combination of things I found on the net, In the head (Clientapp is the name of my form)

<script type="text/javascript" language="javascript">
function Add1(Clientapp) {
var a, b, c;
a = document.Clientapp.BondRepaymentTextBox.Value;
b = document.Clientapp.PropertyRentalTextBox.Value;
c = eval(a) + eval(b);
document.Clientapp.ceTotalTextBox.Value = c;
}
</script>

Code behind on Page Load

PropertyRentalTextBox.Attributes["onchange"] = "javascript: Changed( this );";