Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, [A1]) Is Nothing Then Exit Sub
Dim mem
Application.ScreenUpdating = False
Application.EnableEvents = False
Application.Undo
mem = [A1]
Application.Undo
[A1] = Val(Replace(CStr([A1]), ",", ".")) + Val(Replace(CStr(mem), ",", "."))
Application.EnableEvents = True
End Sub