What was the change in service cost in 2018 from 2017?
{"Cost (income): -- Service cost": {"2019": 41, "2018": 39, "2017": 34}, "Cost (income): -- Interest cost": {"2019": 347, "2018": 196, "2017": 201}, "Cost (income): -- Amortization of prior service credit": {"2019": -191, "2018": -373, "2017": -496}, "Cost (income): -- Net actuarial loss": {"2019": 717, "2018": 25, "2017": 398}, "Cost (income): -- Special termination benefits": {"2019": "\u2014", "2018": 3708, "2017": 591}, "Cost (income): -- Total net periodic postretirement benefit cost": {"2019": 914, "2018": 3595, "2017": 728}, "Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income: -- Prior service cost": {"2019": "\u2014", "2018": "\u2014", "2017": "\u2014"}, "Reversal of amortization items: -- Prior service credit": {"2019": 191, "2018": 373, "2017": 496}, "Reversal of amortization items: -- Total recognized in other comprehensive income": {"2019": 191, "2018": 373, "2017": 496}, "Reversal of amortization items: -- Total recognized in net periodic postretirement cost and other comprehensive income": {"2019": 1105, "2018": 3968, "2017": 1224}}
cost_income_service_cost_2018 = df["Cost (income): -- Service cost"]["2018"]
cost_income_service_cost_2017 = df["Cost (income): -- Service cost"]["2017"]
answer = cost_income_service_cost_2018 - cost_income_service_cost_2017