Adding numbers in a cell

Private Sub Worksheet_Change(ByVal Target As Range)
Dim cell As Range, x, y
Set cell = Intersect(Target, [A1])
If cell Is Nothing Then Exit Sub
With Application
    .EnableEvents = False
    x = cell
    .Undo
    y = cell
    On Error Resume Next
    cell = x + y
    On Error GoTo 0
    .EnableEvents = True
End With
End Sub

Public lastvalue

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address <> "$A$1" Then Exit Sub
If Target = "" Then Exit Sub
If IsNumeric(Target) And IsNumeric(lastvalue) Then
    Application.EnableEvents = False
        Target = Target + lastvalue
    Application.EnableEvents = True
End If
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Address = "$A$1" Then lastvalue = Target
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
Dim cell As Range, x, y
Set cell = Intersect(Target, [A1])
If cell Is Nothing Then Exit Sub
With Application
    .EnableEvents = False
    If Target.Address <> "$A$1" Then
        MsgBox "Change A1 with a single input only."
        .Undo
        GoTo e
    End If
    x = cell
    If Not IsNumeric(x) Or x = "" Then GoTo e
    .Undo
    y = cell
    If IsNumeric(y) Then
        cell = x + y
    Else
        cell = x
    End If
e:    .EnableEvents = True
End With
End Sub