Forms Data Controls :: Access Footer Template Control In DataControlRowType.DataRow?

I have one grid which have 5 columns 1 for subject,2,3,4 for marks which have textbox to insert obtained marks,and last column have one lable to display total of (2+3+4),now i want total value on textbox event,when user enter any value to any column of textbox i need to display total of all three columns in total column. that things i did using javascript now i have one grand total text box in footer template according to sum of total i need to display grand total.so how can i get that footer template textbox in rowtype of DataControlRowType.DataRow.JAVASCRIPT :

<script language="javascript" type="text/javascript">
function sum(m1, m2, m3, txtTotal,txtFTotal) {
var mark1 = document.getElementById(m1).value;

[code]...