<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def nm_width(center, width, units="wn") -> float: """Given a center and width, in energy units, get back a width in nm. Parameters center : number Center (in energy units). width : number Width (in energy units). units : string (optional) Input units. Default is wn. Returns ------- number Width in nm. """

    red = wt_units.converter(center - width / 2., units, "nm")
    blue = wt_units.converter(center + width / 2., units, "nm")
    return red - blue