What is the change in Tax credits from December 31, 2018 and 2019?
{"Federal statutory tax rate": {"2019": 21.0, "2018": 21.0}, "State taxes": {"2019": -4.5, "2018": 4.4}, "Non deductible expenses": {"2019": -0.3, "2018": -0.6}, "Tax credits": {"2019": 4.0, "2018": 4.6}, "Expired tax credit": {"2019": -1.3, "2018": -3.9}, "Deferred tax adjustment": {"2019": -4.8, "2018": "\u2014"}, "Stock based compensation": {"2019": 1.9, "2018": 0.8}, "Valuation allowance": {"2019": 3.2, "2018": -167.0}, "Contingent purchase revaluation": {"2019": "\u2014", "2018": -1.0}, "Other": {"2019": -0.3, "2018": -0.1}, "": {"2019": 18.9, "2018": -141.8}}
tax_credits_2019 = df["Tax credits"]["2019"]
tax_credits_2018 = df["Tax credits"]["2018"]
answer = tax_credits_2019 - tax_credits_2018