What is the gross margin percentage change from 2017 to 2018?
{"": {"2019": "", "2018": "(In thousands, except percentage data)", "2017": ""}, "Cost of revenue": {"2019": 704535, "2018": 717118, "2017": 731453}, "Gross margin percentage": {"2019": 29.5, "2018": 32.3, "2017": 29.6}}
gross_margin_2018 = df["Gross margin percentage"]["2018"]
gross_margin_2017 = df["Gross margin percentage"]["2017"]
answer = gross_margin_2018 - gross_margin_2017