regarding SQL and ASP

Question

i have a table wchich has two columns named as 'sl.no.' and  'total'

and it has 5 values in rows like 

    sl.no     total

      1           3

      2           5

      3           2                         

      4           3

      5           4

now i want to add only the rows starts from sl.no -2 to sl.no -4

 

i an using this code but its not working please let me out

object sumobject 

for (p = 2; p <= 4; p++)

{

DataTable dt;

dt = dsu.Tables[username.Text];

sumobject = dt.Compute("sum(total)", "sl.no=" + p + "");

}

reasoning0.Text = sumobject.ToString();