Beginner error unrecognized expression

hi this is my queries
{"$push": {f"avg_under_water": {"$avg": f"$event_data.under_water"},
f"$event_data.count": f"$event_data.under_water"}}

and i get error of Unrecognized expression , because f"$event_data.count" ,
why i cannot evaluate two value (count ,under water) and put it in the dict ?

what is the right way to do this ?

thanks very much