What is the change in Worldwide Semiconductor Industry Sales from Year Ended December 31, 2018 to 2019?
{"": {"Worldwide Semiconductor Industry Sales (1)": "(in billions)", "Percentage Change": ""}, "2019": {"Worldwide Semiconductor Industry Sales (1)": 412.1, "Percentage Change": -12.1}, "2018": {"Worldwide Semiconductor Industry Sales (1)": 468.8, "Percentage Change": 13.7}, "2017": {"Worldwide Semiconductor Industry Sales (1)": 412.2, "Percentage Change": 21.6}, "2016": {"Worldwide Semiconductor Industry Sales (1)": 338.9, "Percentage Change": 1.1}, "2015": {"Worldwide Semiconductor Industry Sales (1)": 335.2, "Percentage Change": -0.2}}
worldwide_semiconductor_industry_sales_2019 = df["2019"]["Worldwide Semiconductor Industry Sales (1)"]
worldwide_semiconductor_industry_sales_2018 = df["2018"]["Worldwide Semiconductor Industry Sales (1)"]
answer = worldwide_semiconductor_industry_sales_2019 - worldwide_semiconductor_industry_sales_2018