Calculation AZ by equation

Hello,

Im trying to make sure my ACU unit that controlls the dishpointing works perfect, so I am trying to use math in order to calculate the AZ pointing angle using the equation

A= 180 + arctan [ tan(S-N)/sinL ] where S is the satellite long, N the site long and L the site LAT (all in degrees)

My question is a bit simple for you I guess, in order to use this equation I need to have the LONG part in degrees west. How do I transform long from East to West?

i.e. In Athens we have LAT LONG 37 N 23 E

What would that be in degrees west ?

LAT LONG 37 N  (=?) W    ???

Im a bit confused on how to calculate this one

Thanks,

Xband