Perhaps:
Dim rngCell As Range, lngRow As Long, bMod As Byte
lngRow = 1 'start row
bMod = lngRow Mod 2
For Each rngCell in Range(Cells(lngRow,"A"),Cells(Rows.Count,"A").End(xlUp))
If rngCell.Row Mod 2 = bMod Then rngCell.Value = -rngCell.Value
Next rngCell
A "negative" row is deemed as one that equates to the Odd/Even status of the first row in the range being processed... ie if first row is 2 then Even rows should be set to opposite sign, if first row is 1 then Odd rows should be set to opposite sign.
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