Sum two rows in a range

[GIS2013]

Hi. I have a "big" range and a smaller one in which each cell is the sum of two cells in the big range. It's more simple in the file attached. I was thinking about something like this, any modification/other suggestion?

Private Sub test()
Dim a As Range, b As Range, c As Byte
    With Worksheets("s")
    Set a = .Range("a2:e6")
        For Each b In a
        b.Value = Application.WorksheetFunction.Sum(.Cells(b.Offset(9), b.Offset(11)))
        Next b
    End With
End Sub