What was the percentage change in gross profit of net sales between 2017 and 2018?
{"Net sales": {"2019": 100.0, "2018": 100.0, "2017": 100.0}, "Cost of sales": {"2019": 45.2, "2018": 39.2, "2017": 48.4}, "Gross profit": {"2019": 54.8, "2018": 60.8, "2017": 51.6}, "Research and development": {"2019": 15.4, "2018": 13.3, "2017": 16.0}, "Selling, general and administrative": {"2019": 12.8, "2018": 11.4, "2017": 14.7}, "Amortization of acquired intangible assets": {"2019": 12.6, "2018": 12.2, "2017": 9.9}, "Special charges and other, net": {"2019": 0.6, "2018": 0.4, "2017": 2.9}, "Operating income": {"2019": 13.4, "2018": 23.5, "2017": 8.1}}
gross_profit_2018 = df["Gross profit"]["2018"]
gross_profit_2017 = df["Gross profit"]["2017"]
answer = gross_profit_2018 - gross_profit_2017