What is the change in Statutory U.S. Federal tax between 2018 and 2019?
{"Statutory U.S. Federal tax": {"December 27, 2019 ": 6805, "December 28, 2018 ": 5847, "December 29, 2017": 6443}, "Differences due to: -- State and local taxes, net of federal benefit ": {"December 27, 2019 ": 2078, "December 28, 2018 ": 1906, "December 29, 2017": 1112}, "Differences due to: -- Change in valuation allowance ": {"December 27, 2019 ": 95, "December 28, 2018 ": 523, "December 29, 2017": 289}, "Differences due to: -- Impact of the Tax Act ": {"December 27, 2019 ": "\u2014", "December 28, 2018 ": "\u2014", "December 29, 2017": -3573}, "Differences due to: -- Stock compensation ": {"December 27, 2019 ": -676, "December 28, 2018 ": -197, "December 29, 2017": 162}, "Differences due to: -- Other ": {"December 27, 2019 ": -92, "December 28, 2018 ": -637, "December 29, 2017": -391}, "Differences due to: -- Income tax expense": {"December 27, 2019 ": 8210, "December 28, 2018 ": 7442, "December 29, 2017": 4042}}
statutory_us_federal_tax_december_27_2019 = df["Statutory U.S. Federal tax"]["December 27, 2019 "]
statutory_us_federal_tax_december_28_2018 = df["Statutory U.S. Federal tax"]["December 28, 2018 "]
answer = statutory_us_federal_tax_december_27_2019 - statutory_us_federal_tax_december_28_2018