What was the change in service cost from 2017 to 2018?
{"Service cost": {"2019": 162, "2018": 250, "2017": 313}, "Interest cost": {"2019": 1740, "2018": 1687, "2017": 1901}, "Expected return on plan assets": {"2019": -375, "2018": -926, "2017": -843}, "Recognized actuarial loss": {"2019": 777, "2018": 1392, "2017": 1182}, "Net periodic benefit cost": {"2019": 2304, "2018": 2403, "2017": 2553}}
service_cost_2018 = df["Service cost"]["2018"]
service_cost_2017 = df["Service cost"]["2017"]
answer = service_cost_2018 - service_cost_2017