What is the change in the Tax credits from 2018 to 2019?
{"Tax credits": {"December 28, 2019": 44696, "December 29, 2018": 39586}, "Inventory reserve": {"December 28, 2019": 12350, "December 29, 2018": 10850}, "Other reserves and accruals": {"December 28, 2019": 5852, "December 29, 2018": 5398}, "Non-statutory stock options": {"December 28, 2019": 2982, "December 29, 2018": 2722}, "Depreciation and amortization": {"December 28, 2019": 27758, "December 29, 2018": 1979}, "Net operating loss carryforwards": {"December 28, 2019": 21410, "December 29, 2018": 61275}, "Gross deferred tax assets": {"December 28, 2019": 115048, "December 29, 2018": 121810}, "Valuation allowance": {"December 28, 2019": -36604, "December 29, 2018": -34037}, "Total deferred tax assets": {"December 28, 2019": 78444, "December 29, 2018": 87773}, "Acquired intangibles and fixed assets": {"December 28, 2019": -13997, "December 29, 2018": -12667}, "Unrealized investment gains": {"December 28, 2019": -106, "December 29, 2018": -107}, "Tax on undistributed earnings": {"December 28, 2019": -75, "December 29, 2018": -53}, "Total deferred tax liabilities": {"December 28, 2019": -14178, "December 29, 2018": -12827}, "Net deferred tax assets": {"December 28, 2019": 64266, "December 29, 2018": 74946}}
tax_credits_december_28_2019 = df["Tax credits"]["December 28, 2019"]
tax_credits_december_29_2018 = df["Tax credits"]["December 29, 2018"]
answer = tax_credits_december_28_2019 - tax_credits_december_29_2018