What is the difference in tax credits between 2018 and 2019?
{"Tax at federal statutory rate": {"2019": 21.0, "2018": 21.0, "2017": 35.0}, "State taxes, net of federal effect": {"2019": -0.3, "2018": -0.1, "2017": -5.4}, "Foreign rate differential": {"2019": -6.0, "2018": -3.9, "2017": -9.3}, "Tax credits": {"2019": 1.6, "2018": 6.3, "2017": 4.1}, "Domestic production activities deduction": {"2019": "\u2014", "2018": "\u2014", "2017": -3.5}, "Stock-based compensation": {"2019": -1.0, "2018": -4.9, "2017": -5.3}, "Change in prior year reserves": {"2019": -1.5, "2018": -0.1, "2017": -2.0}, "Change in valuation allowance": {"2019": -13.4, "2018": -15.2, "2017": -35.2}, "Effect of change in tax rate due to Tax Act": {"2019": "\u2014", "2018": "\u2014", "2017": -23.4}, "Other (1)": {"2019": -4.2, "2018": -4.0, "2017": 2.6}, "Effective tax rate": {"2019": -3.8, "2018": -0.9, "2017": -42.4}}
tax_credits_2018 = df["Tax credits"]["2018"]
tax_credits_2019 = df["Tax credits"]["2019"]
answer = tax_credits_2018 - tax_credits_2019