Field calc help

I'm trying to populate a field with an already defined area but only if the Updat_HYGR variable is = to 'A'

i have a column with letters A-D (Updat_HYGR), a Column with areas in sqmi (Sq_Miles) and a new column A_SqMi that i only want populated with the area if the Updat_HYGRP = 'A'

i just started using python but my statement looks like this

def Reclass(A_SqMi):
if (Updat_HYGR == 'A'):
return Sq_Miles



Reclass(!A_SqMi!)


Any help would be appreciated