What is the difference in net rent expense between 2017 and 2018?
{"Minimum rentals": {"2019": 184587, "2018": 184106, "2017": 185696}, "Contingent rentals": {"2019": 2255, "2018": 2221, "2017": 2419}, "Total rent expense": {"2019": 186842, "2018": 186327, "2017": 188115}, "Less rental expense on subleased properties": {"2019": -170651, "2018": -162640, "2017": -145728}, "Net rent expense": {"2019": 16191, "2018": 23687, "2017": 42387}}
net_rent_2017 = df["Net rent expense"]["2017"]
net_rent_2018 = df["Net rent expense"]["2018"]
answer = net_rent_2017 - net_rent_2018