What was the change in the cost of sales between 2017 and 2018?
{"Cost of sales (1)": {"2019": 14.9, "2018": 13.8, "2017": 18.7}, "Research and development": {"2019": 72.0, "2018": 42.5, "2017": 46.8}, "Selling, general and administrative": {"2019": 62.3, "2018": 36.9, "2017": 62.6}, "Special (income) charges and other, net": {"2019": 17.2, "2018": "\u2014", "2017": "\u2014"}, "Pre-tax effect of share-based compensation": {"2019": 166.4, "2018": 93.2, "2017": 128.1}, "Income tax benefit": {"2019": 35.5, "2018": 28.3, "2017": 44.2}, "Net income effect of share-based compensation": {"2019": 130.9, "2018": 64.9, "2017": 83.9}}
cost_2018 = df["Cost of sales (1)"]["2018"]
cost_2017 = df["Cost of sales (1)"]["2017"]
answer = cost_2018 - cost_2017