Sub echange()
Columns(4).Insert
Range("d2:d" & Cells.Find("*", , , , xlByRows, xlPrevious).Row).Formula = "=MAX(0,b2-c2)"
Range("d2:d" & Cells.Find("*", , , , xlByRows, xlPrevious).Row).Copy
[b2].PasteSpecial Paste:=xlPasteValues
Columns(4).Delete
End Sub