What was the change in product development between 2018 and 2019?
{"Cost of revenuesproduct sales: Software royalties, amortization, and intellectual property licenses": {"2019": 19, "2018": 13, "2017": 10}, "Cost of revenuessubscription, licensing, and other revenues: Game Operations and Distribution Costs": {"2019": 1, "2018": 2, "2017": 1}, "Cost of revenuessubscription, licensing, and other revenues: Software royalties, amortization, and intellectual property licenses": {"2019": 1, "2018": 3, "2017": 3}, "Product development": {"2019": 53, "2018": 61, "2017": 57}, "Sales and marketing": {"2019": 10, "2018": 15, "2017": 15}, "General and administrative": {"2019": 82, "2018": 115, "2017": 92}, "Share-based compensation expense before income taxes": {"2019": 166, "2018": 209, "2017": 178}, "Income tax benefit": {"2019": -29, "2018": -46, "2017": -34}, "Total share-based compensation expense, net of income tax benefit": {"2019": 137, "2018": 163, "2017": 144}}
product_development_2019 = df["Product development"]["2019"]
product_development_2018 = df["Product development"]["2018"]
answer = product_development_2019 - product_development_2018