Private Sub Worksheet_Change(ByVal Target As Excel.Range)
    Dim sum#, c
    With Target
        On Error GoTo err_
        Application.EnableEvents = False
        If .Address(0, 0) = "A1" Then
            For Each c In Split(Trim([A1]))
               If c = "" Then c = 0
                If Not IsNumeric(c) Then Error.Value
                sum = sum + c
            Next
            [A2] = sum
        End If
    End With
err_:
    If Err Then MsgBox "Ошибка ввода": Err.Clear
    Application.EnableEvents = True
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Dim i&, m, n&
    If Target.Address(0, 0) = "A2" Then [A1].Activate Else Exit Sub
    [A1].Font.FontStyle = "обычный"
    m = Split([A1])
    For i = 0 To UBound(m)
        n = n + Len(m(i)) + 1
        [A1].Characters(n, -Len(m(i))).Font.ColorIndex = i Mod 2 + 9
    Next
    SendKeys "{F2} "
End Sub