Private Sub Worksheet_SelectionChange(ByVal Target As Range)
a = Target.Row
If Target.Column = 4 Then
Target = IIf(Target = "X", "", "X")
If IsEmpty(Cells(a, 3).Value) Then
Cells(a, 3).Value = IIf(Target = "X", "1", Cells(a, 3).Value)
End If
Cells(a, 3).Select
Exit Sub
End If
End Sub