Dim n_
Private Sub Worksheet_Change(ByVal Target As Range)
     If Target.Address(0, 0) = "A1" Then
         Application.EnableEvents = 0
         On Error Resume Next
         Target = WorksheetFunction.Sum(n_, Target)
         If Err.Number <> 0 Then Target = n_
         On Error GoTo 0
         n_ = Target
         Target.Select
         Application.EnableEvents = 1
     End If
End Sub
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
     If Target.Address(0, 0) = "A1" Then n_ = Target
End Sub

Dim i
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
        With Target
           If .Address(False, False) = "A1" Then
              If IsNumeric(.Value) Then
                 Application.EnableEvents = False
                  i = Selection.Value
                 Application.EnableEvents = True
              End If
           End If
        End With
End Sub

Private Sub Worksheet_Change(ByVal Target As Excel.Range)
        With Target
           If .Address(False, False) = "A1" Then
              If IsNumeric(.Value) Then
                 Application.EnableEvents = False
                 s = Range("A1").Value
                 Range("A1").Value = Range("A1").Value + i
                 [a1].Select
                 i = Selection.Value
                 Application.EnableEvents = True
              End If
           End If
        End With
End Sub