Just a lil bit extension of ajryan's .. 
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Range("j:k"), Target) Is Nothing Then
If ((Range("j" & Target.Row) = "") + (Range("K" & Target.Row) = "")) Then Range("l" & Target.Row) = ""
End If
End Sub
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Address = "$J$" & Target.Row & ":$K$" & Target.Row And _
((Range("j" & Target.Row) <> "") * (Range("K" & Target.Row) <> "")) _
Then Range("L" & Target.Row).Formula = _
"=I" & Target.Row & "&"" ""&J" & Target.Row & " &"" ""&K" & Target.Row & "&"" - ""&E" & Target.Row & "&"" ""&"" ""&F" & Target.Row
End Sub
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