What is the change in Per-unit royalty revenue between 2018 and 2019?
{"Revenues:      -- Fixed fee license revenue": {"2019": 35.1, "2018": 75.3, "2017": 36.0}, "Revenues:      -- Per-unit royalty revenue": {"2019": 64.0, "2018": 24.3, "2017": 61.4}, "Revenues:      -- Total royalty and license revenue": {"2019": 99.1, "2018": 99.6, "2017": 97.4}, "Revenues:      -- Development, services, and other": {"2019": 0.9, "2018": 0.4, "2017": 2.6}, "Revenues:      -- Total revenues": {"2019": 100.0, "2018": 100.0, "2017": 100.0}, "Costs and expenses: -- Cost of revenues": {"2019": 0.5, "2018": 0.2, "2017": 0.6}, "Costs and expenses: -- Sales and marketing": {"2019": 17.9, "2018": 5.5, "2017": 38.6}, "Costs and expenses: -- Research and development": {"2019": 21.8, "2018": 8.8, "2017": 33.6}, "Costs and expenses: -- General and administrative": {"2019": 119.4, "2018": 37.7, "2017": 152.4}, "Costs and expenses: -- Restructuring costs": {"2019": "\u2014", "2018": "\u2014", "2017": 4.6}, "Costs and expenses: -- Total costs and expenses": {"2019": 159.6, "2018": 52.2, "2017": 229.8}, "Costs and expenses: -- Operating income (loss)": {"2019": -59.6, "2018": 47.8, "2017": -129.8}, "Costs and expenses: -- Interest and other income": {"2019": 5.0, "2018": 1.7, "2017": 1.0}, "Costs and expenses: -- Other expense": {"2019": 0.2, "2018": -0.2, "2017": 0.9}, "Costs and expenses: -- Income (loss) before provision for income taxes": {"2019": -54.4, "2018": 49.3, "2017": -127.9}, "Costs and expenses: -- Provision for income taxes": {"2019": -1.3, "2018": -0.4, "2017": -1.4}, "Costs and expenses: -- Net income (loss)": {"2019": -55.7, "2018": 48.9, "2017": -129.3}}
per_unit_royalty_revenue_2019 = df["Revenues:      -- Per-unit royalty revenue"]["2019"]
per_unit_royalty_revenue_2018 = df["Revenues:      -- Per-unit royalty revenue"]["2018"]
answer = per_unit_royalty_revenue_2019 - per_unit_royalty_revenue_2018