DATATABLE problem retrieving the value of a cell

[anon]

I have a datatable of values and I am trying to obtain the values of a range
of the columns.

Imagine I have a table defined with columns headed "A" through to "Z"

Now I decide that I want to count up the amounts between columns E to M
Note:All the columns are defined as decimal.
I have a main datatable defined at dtMain with all the columns and data in
it.
The code I am using at the moment is:

Private Sub cmdCalculate_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) _
Handles cmdCalculate.Click
Dim dr As DataRow
Dim OrdS As Integer
Dim OrdF As Integer

'find the ordinal start and end for the addition
OrdS = dtMain.Columns("E").Ordinal
OrdF = dtMain.Columns("M").Ordinal

For Each dr In dtMain.Rows
Debug.WriteLine("----------------------------------------")
Dim sumcol As Decimal

For i = OrdS To OrdF
Debug.WriteLine(dtMain.Columns(i).Caption & " - " &
dtMain.Columns(i).ToString()) 'I KNOW THIS IS WRONG

'sumcol =sumcol + [????????????????????????]

Next
debug.writeline(sumcol)
Next

End Sub

please can anyone advise how I can obtain the contents of the cell in the
dtmain table so that I can achieve a calculation.

Thanks in advance

Terry