Private Sub Worksheet_Change(ByVal Target As Excel.Range)
    If Target.Row = 1 And (Target.Column = 1 Or Target.Column = 2) Then
    'new val
    a1 = Range("A1").Value
    b1 = Range("B1").Value
    newVal = Range("D1").Value

    'disable events and undo
    Application.EnableEvents = False
    Application.Undo
    'old val
    oldVal = Range("D1").Value

    'diff between new and old val
    diff = newVal - oldVal

    Range("A1").Value = a1
    Range("B1").Value = b1
    Range("E1").Value = diff

    're-enable events
    Application.EnableEvents = True
End If
End Sub