Try this then:
Sub notepage()
Dim osld As Slide
Dim oNum As Shape
Dim x As Long
For Each osld In ActivePresentation.Slides
For Each oNum In osld.NotesPage.Shapes
If oNum.Type = msoPlaceholder Then
If oNum.PlaceholderFormat.Type = ppPlaceholderSlideNumber Then
If osld.SlideIndex / 2 = osld.SlideIndex \ 2 Then 'it's even do not increment
oNum.TextFrame2.TextRange = "FG" & CStr(x)
Else ' it's odd
'increment number
x = x + 1
oNum.TextFrame2.TextRange = CStr(x)
End If
End If
End If
Next oNum
Next osld
End Sub