What was the change in operating income between 2017 and 2018?
{"Revenues": {"2019": 1177.2, "2018": 1114.0, "2017": 1051.6}, "Expenses: -- Operating expenses": {"2019": 646.0, "2018": 625.4, "2017": 569.5}, "Expenses: -- Depreciation and amortization": {"2019": 236.2, "2018": 217.0, "2017": 206.5}, "Expenses: -- Transition and integration costs": {"2019": 5.4, "2018": 6.6, "2017": 13.1}, "Expenses: -- Total expenses": {"2019": 887.6, "2018": 849.0, "2017": 789.1}, "Expenses: -- Operating income": {"2019": 289.6, "2018": 265.0, "2017": 262.5}, "Expenses: -- Operating margin": {"2019": 24.6, "2018": 23.8, "2017": 25.0}, "Expenses: -- Interest expense, net": {"2019": -63.5, "2018": -51.7, "2017": -57.5}, "Expenses: -- Other expense, net": {"2019": -1.4, "2018": -7.1, "2017": -12.6}, "Expenses: -- Earnings before income taxes and equity in losses of unconsolidated affiliates": {"2019": 224.7, "2018": 206.2, "2017": 192.4}, "Expenses: -- Income tax expense (benefit)": {"2019": 41.9, "2018": 37.7, "2017": -61.8}, "Expenses: -- Earnings before equity in losses of unconsolidated affiliates": {"2019": 182.8, "2018": 168.5, "2017": 254.2}, "Expenses: -- Equity in losses of unconsolidated affiliates, net of tax": {"2019": -74.0, "2018": "\u2014", "2017": "\u2014"}, "Expenses: -- Net earnings": {"2019": 108.8, "2018": 168.5, "2017": 254.2}, "Net earnings per share attributable to Black Knight common shareholders: -- Diluted": {"2019": 0.73, "2018": 1.14, "2017": 1.47}, "Weighted average shares of common stock outstanding: -- Diluted": {"2019": 148.6, "2018": 148.2, "2017": 152.4}}
expenses__operating_income_2018 = df["Expenses: -- Operating income"]["2018"]
expenses__operating_income_2017 = df["Expenses: -- Operating income"]["2017"]
answer = expenses__operating_income_2018 - expenses__operating_income_2017