What was the change in services revenue between 2018 and 2019?
{"Services ": {"2019": 59545, "2018": 64476}, "Software and other  ": {"2019": 3788, "2018": 5073}, "             Total revenue ": {"2019": 63333, "2018": 69549}}
services_2019 = df["Services "]["2019"]
services_2018 = df["Services "]["2018"]
answer = services_2019 - services_2018