pre_text: Net periodic benefit expense for our post-retirement benefit plans includes the following components: We report service costs for our Combined Pension Plan and post-retirement benefit plans in cost of services and products and selling, general and administrative expenses in our consolidated statements of operations for the years ended December 31, 2019, 2018 and 2017. Additionally, a portion of the service cost is also allocated to certain assets under construction, which are capitalized and reflected as part of property, plant and equipment in our consolidated balance sheets. The remaining components of net periodic benefit expense (income) are reported in other income, net in our consolidated statements of operations. As a result of ongoing efforts to reduce our workforce, we recognized a one-time charge in 2019 of $6 million and in 2018 of $15 million for special termination benefit enhancements paid to certain eligible employees upon voluntary retirement.
table: Post-Retirement Plans
table: Years Ended December 31,
table: 2019 2018 2017
table: (Dollars in milions)
table: Service cost $15 18 18
table: Interest cost 110 97 100
table: Expected return on plan assets (1) (1) (2)
table: Recognition of prior service cost 16 20 20
table: Net periodic post-retirement benefit expense $140 134 136
question: Analyse this data from a financial earnings document. What is the sum of service costs in 2017, 2018 and 2019?