What was the change in the cost of revenue between 2017 and 2018?
{"Total revenue": {"2019": 529646.0, "2018": 414673.0, "2017": 325887.0, "2016": 263865.0, "2015": 173457.0}, "Cost of revenue (exclusive of depreciation and amortization)": {"2019": 248580.0, "2018": 160439.0, "2017": 89708.0, "2016": 79145.0, "2015": 36506.0}, "Total costs and expenses": {"2019": 408693.0, "2018": 261883.0, "2017": 180288.0, "2016": 144054.0, "2015": 80351.0}, "Operating profit": {"2019": 120953.0, "2018": 152790.0, "2017": 145599.0, "2016": 119811.0, "2015": 93106.0}, "Total other income (expense), net": {"2019": -32105.0, "2018": -19276.0, "2017": -6931.0, "2016": 4653.0, "2015": 713.0}, "Income before income tax expense": {"2019": 88848.0, "2018": 133514.0, "2017": 138668.0, "2016": 124464.0, "2015": 93819.0}, "Income tax expense (benefit)": {"2019": -7125.0, "2018": 5534.0, "2017": "\u2014", "2016": "\u2014", "2015": "\u2014"}, "Net income": {"2019": 95973.0, "2018": 127980.0, "2017": 138668.0, "2016": 124464.0, "2015": 93819.0}, "Net income attributable to noncontrolling interests": {"2019": 63993.0, "2018": 103724.0, "2017": "N/A", "2016": "N/A", "2015": "N/A"}, "Net income attributable to GreenSky, Inc.": {"2019": 31980.0, "2018": 24256.0, "2017": "N/A", "2016": "N/A", "2015": "N/A"}, "Earnings per share of Class A common stock(1): -- Basic": {"2019": 0.52, "2018": 0.43, "2017": "N/A", "2016": "N/A", "2015": "N/A"}, "Earnings per share of Class A common stock(1): -- Diluted": {"2019": 0.49, "2018": 0.41, "2017": "N/A", "2016": "N/A", "2015": "N/A"}}
cost_of_revenue_2018 = df["Cost of revenue (exclusive of depreciation and amortization)"]["2018"]
cost_of_revenue_2017 = df["Cost of revenue (exclusive of depreciation and amortization)"]["2017"]
answer = cost_of_revenue_2018 - cost_of_revenue_2017