Can assign to XtraReport dynamic data calculate the total ?

Question

Suppose I have the following code:

[CODE]

private void button1_Click(object sender, EventArgs e)

{

DataTable dt = CreateTableBill(10);

rptReport rpt = new rptReport();

rpt.DataSource = dt;

rpt.ShowPreview();

}

private static DataTable CreateTable(int RowCount)

{

DataTable tbl = new DataTable();

tbl.Columns.Add("ID", typeof(int));

tbl.Columns.Add("Product", typeof(string));

tbl.Columns.Add("Number", typeof(int));

tbl.Columns.Add("UnitPrice", typeof(int));

for (int i = 0; i < RowCount; i++)

tbl.Rows.Add(new object[] { i + 1, String.Format("Product {0}", i), 3 - i, 2.5 * 100 });

return tbl;

}

[/CODE]

to turn on their schools dt can be assigned to schools dt

XrTableCell1 = [Product];

XrTableCell2 = [Number];

XrTableCell3 = [UnitPrice];

XrTableCell4 = [Number] * [UnitPrice];

XrTableCell5 = Sum ([Number] * [UnitPrice]);

when you open the report in design mode, I want to calculate the sum of XrTableCell5, I have to XrTableCell5 under DataBindings-> Text: assignment: Sum ([Number] * [UnitPrice]), but the result None, why not calculate the total be ? you see attach file http://www.mediafire.com/view/s1fs5fhxnz4z1f2/Sumbill.jpg