What was the change in service costs between 2017 and 2018?
{"Service costs": {"2019": 1.5, "2018": 2.2, "2017": 1.4}, "Interest costs": {"2019": 1.1, "2018": 1.0, "2017": 1.0}, "Amortization of actuarial loss": {"2019": 0.4, "2018": 0.8, "2017": "\u2014"}, "Settlements": {"2019": "\u2014", "2018": "\u2014", "2017": 0.5}, "Net pension period cost": {"2019": 3.0, "2018": 4.0, "2017": 2.9}}
service_costs_2018 = df["Service costs"]["2018"]
service_costs_2017 = df["Service costs"]["2017"]
answer = service_costs_2018 - service_costs_2017