What is the change in Selling, general and administrative expense from 2018 to 2019?
{"Cost of services ": {"2019": 10655.0, "2018": 10859.0, "2019 vs. 2018": -204.0, "": -1.9}, "Cost of wireless equipment ": {"2019": 4733.0, "2018": 4560.0, "2019 vs. 2018": 173.0, "": 3.8}, "Selling, general and administrative expense ": {"2019": 8188.0, "2018": 7689.0, "2019 vs. 2018": 499.0, "": 6.5}, "Depreciation and amortization expense ": {"2019": 4105.0, "2018": 4258.0, "2019 vs. 2018": -153.0, "": -3.6}, "Total Operating Expenses ": {"2019": 27681.0, "2018": 27366.0, "2019 vs. 2018": 315.0, "": 1.2}}
selling_general_and_administrative_expense_2019 = df["Selling, general and administrative expense "]["2019"]
selling_general_and_administrative_expense_2018 = df["Selling, general and administrative expense "]["2018"]
answer = selling_general_and_administrative_expense_2019 - selling_general_and_administrative_expense_2018