What was the change in the cost of services from 2018 to 2019?
{"Cost of services ": {"2019": 31772.0, "2018": 32185.0, "2019 vs. 2018": -413.0, "": -1.3}, "Cost of wireless equipment ": {"2019": 22954.0, "2018": 23323.0, "2019 vs. 2018": -369.0, "": -1.6}, "Selling, general and administrative expense": {"2019": 29896.0, "2018": 31083.0, "2019 vs. 2018": -1187.0, "": -3.8}, "Depreciation and amortization expense ": {"2019": 16682.0, "2018": 17403.0, "2019 vs. 2018": -721.0, "": -4.1}, "Media goodwill impairment ": {"2019": 186.0, "2018": 4591.0, "2019 vs. 2018": -4405.0, "": -95.9}, "Consolidated Operating Expenses": {"2019": 101490.0, "2018": 108585.0, "2019 vs. 2018": -7095.0, "": -6.5}}
cost_of_services_2019 = df["Cost of services "]["2019"]
cost_of_services_2018 = df["Cost of services "]["2018"]
answer = cost_of_services_2019 - cost_of_services_2018