This is a bit messay but you do ...
add a textbox to the report detail line.(txt1)
Set the controlsource to =1 and set the Runningsum property to Over All.
Set the visible property to No.
add another textbox to the report detail line (this is your visible number)
set the controlsource to
=(txt1+1)\2 (note \ NOT /)
set the HideDuplicates property to Yes