What was the increase / (decrease) in the inventory valuation reserve from 2018 to 2019?
{"Deferred tax assets: -- Accrued liabilities": {"2019": 3944, "2018": 2720}, "Deferred tax assets: -- Allowance for doubtful accounts": {"2019": 120, "2018": 143}, "Deferred tax assets: -- Inventory valuation reserve": {"2019": 41, "2018": 20}, "Deferred tax assets: -- Federal losses and credit carryforwards": {"2019": 45227, "2018": 42713}, "Deferred tax assets: -- Foreign net operating losses": {"2019": 730, "2018": 623}, "Deferred tax assets: -- State losses and credit carryforwards": {"2019": 9886, "2018": 9592}, "Deferred tax assets: -- Deferred revenue": {"2019": 488, "2018": 652}, "Deferred tax assets: -- Goodwill and other intangible assets": {"2019": "\u2014", "2018": 286}, "Deferred tax assets: -- Other": {"2019": 65, "2018": 96}, "Deferred tax assets: -- ": {"2019": 60501, "2018": 56845}, "Deferred tax assets: -- Less: valuation allowance": {"2019": -57852, "2018": -54260}, "Deferred tax assets: -- Total": {"2019": 2649, "2018": 2585}, "Deferred tax liabilities: -- Property and equipment & software amortization": {"2019": -361, "2018": -412}, "Deferred tax liabilities: -- Goodwill and other intangible assets": {"2019": -2706, "2018": -2277}, "Deferred tax liabilities: -- Total": {"2019": -3067, "2018": -2689}, "Deferred tax liabilities: -- Total deferred tax liabilities": {"2019": -418, "2018": -104}}
inventory_valuation_reserve_2019 = df["Deferred tax assets: -- Inventory valuation reserve"]["2019"]
inventory_valuation_reserve_2018 = df["Deferred tax assets: -- Inventory valuation reserve"]["2018"]
answer = inventory_valuation_reserve_2019 - inventory_valuation_reserve_2018