What is the change in Service cost from December 31, 2018 to 2019?
{"Service cost": {"2019": 9.4, "2018": 9.6, "2017": 10.0}, "Interest cost": {"2019": 5.0, "2018": 4.7, "2017": 4.3}, "Expected return on plan assets": {"2019": -6.0, "2018": -6.1, "2017": -5.5}, "Curtailment gain": {"2019": "\u2014", "2018": -0.3, "2017": "\u2014"}, "Actuarial and other loss": {"2019": 15.6, "2018": 6.1, "2017": 1.9}, "Total net periodic pension cost": {"2019": 24.0, "2018": 14.0, "2017": 10.7}, "Weighted average assumptions -- Discount rate used for net periodic pension costs": {"2019": 1.74, "2018": 1.66, "2017": 1.6}, "Weighted average assumptions -- Discount rate used for pension benefit obligations": {"2019": 1.43, "2018": 1.74, "2017": 1.66}, "Weighted average assumptions -- Expected return on plan assets": {"2019": 3.23, "2018": 3.18, "2017": 3.22}, "Weighted average assumptions -- Rate of compensation increase": {"2019": 3.07, "2018": 3.22, "2017": 3.22}}
service_cost_2019 = df["Service cost"]["2019"]
service_cost_2018 = df["Service cost"]["2018"]
answer = service_cost_2019 - service_cost_2018