What was the change in Purchases of property, plant, equipment and software in 2019 from 2018?
{"Net sales": {"2019": 2380.2, "2018": 2157.7, "2017": 1797.6}, "Net sales excluding surcharge revenue (1)": {"2019": 1942.1, "2018": 1792.3, "2017": 1558.4}, "Operating income": {"2019": 241.4, "2018": 189.3, "2017": 121.5}, "Net income": {"2019": 167.0, "2018": 188.5, "2017": 47.0}, "Diluted earnings per share": {"2019": 3.43, "2018": 3.92, "2017": 0.99}, "Purchases of property, plant, equipment and software": {"2019": 180.3, "2018": 135.0, "2017": 98.5}, "Free cash flow (1)": {"2019": -53.7, "2018": 34.7, "2017": -16.8}, "Pounds sold (in thousands) (2)": {"2019": 267536.0, "2018": 265620.0, "2017": 236346.0}}
purchases_2019 = df["Purchases of property, plant, equipment and software"]["2019"]
purchases_2018 = df["Purchases of property, plant, equipment and software"]["2018"]
answer = purchases_2019 - purchases_2018