Private Sub Worksheet_Change() '(ByVal Target As Range)
Dim cel As Range
If Not Intersect(Range("A1:B1"), Target) Is Nothing Then
    If IsNumeric(Range("B1").Value) Then
        For Each cel In Range("A1:A50")
            Application.EnableEvents = False
            If Range("A" & cel.Row) = "" Then
                Range("B" & cel.Row) = Abs(Range("B" & cel.Row).Value)
            Else
                Range("B" & cel.Row) = -Abs(Range("B" & cel.Row).Value)
            End If
        Next cel
            Application.EnableEvents = True
    End If
End If
End Sub