Add Two Number . but Cannot be store value

SqlCommand cmd = new SqlCommand();
cmd.CommandText = "Select FeePaid,Refund From FeeDetails";
cmd.Connection = con;
SqlDataReader dr = cmd.ExecuteReader();
{

if (dr.Read())
{
a = dr[0].ToString();
b = dr[1].ToString();
c = a + b;
Label2.Text = c.ToString();