What is the sum of income tax expense (benefit) in 2018 and 2019?
{"": {"2019": "", "2018": "(Dollars in millions)", "2017": ""}, "Total segment adjusted EBITDA": {"2019": 15987, "2018": 16647, "2017": 12560}, "Depreciation and amortization": {"2019": -4829, "2018": -5120, "2017": -3936}, "Goodwill impairment": {"2019": -6506, "2018": -2726, "2017": "\u2014"}, "Other operating expenses": {"2019": -7216, "2018": -8045, "2017": -6504}, "Share-based compensation": {"2019": -162, "2018": -186, "2017": -111}, "Operating (loss) income": {"2019": -2726, "2018": 570, "2017": 2009}, "Total other expense, net": {"2019": -2040, "2018": -2133, "2017": -1469}, "(Loss) income before income taxes": {"2019": -4766, "2018": -1563, "2017": 540}, "Income tax expense (benefit)": {"2019": 503, "2018": 170, "2017": -849}, "Net (loss) income": {"2019": -5269, "2018": -1733, "2017": 1389}}
income_tax_expense_2019 = df["Income tax expense (benefit)"]["2019"]
income_tax_expense_2018 = df["Income tax expense (benefit)"]["2018"]
answer = income_tax_expense_2019 + income_tax_expense_2018