What is the sum of service costs in 2017, 2018 and 2019?
{"": {"2019": "(Dollars in milions)", "2018": "", "2017": ""}, "Service cost": {"2019": 15, "2018": 18, "2017": 18}, "Interest cost": {"2019": 110, "2018": 97, "2017": 100}, "Expected return on plan assets": {"2019": -1, "2018": -1, "2017": -2}, "Recognition of prior service cost": {"2019": 16, "2018": 20, "2017": 20}, "Net periodic post-retirement benefit expense": {"2019": 140, "2018": 134, "2017": 136}}
service_cost_2017 = df["Service cost"]["2017"]
service_cost_2018 = df["Service cost"]["2018"]
service_cost_2019 = df["Service cost"]["2019"]
answer = service_cost_2017 + service_cost_2018 + service_cost_2019