int count = 0;
for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
{
count = count+ Convert.ToInt32(ds.Tables[0].Rows[i][2].ToString());
}
DataRow dr = ds.Tables[0].NewRow();
dr[0]="";
dr[1] = "Total::";
dr[2]=count.ToString();
ds.Tables[0].Rows.Add(dr);