What is the difference between the total deferred tax in 2018 and 2019?
{"Estimated future value of NOLs -- - Federal": {"2019": 2622, "2018": 2174}, "Estimated future value of NOLs -- - State": {"2019": 869, "2018": 862}, "Research and development tax credits -- - Federal": {"2019": 1207, "2018": 1184}, "Research and development tax credits -- - State": {"2019": 8, "2018": "-"}, "Research and development tax credits -- Share-based compensation": {"2019": 72, "2018": 71}, "Research and development tax credits -- Other, net": {"2019": 177, "2018": 151}, "Research and development tax credits -- Total deferred taxes": {"2019": 4955, "2018": 4442}, "Research and development tax credits -- Valuation allowance": {"2019": -4955, "2018": -4442}, "Research and development tax credits -- Net deferred tax assets": {"2019": "$-", "2018": "$-"}}
total_deferred_tax_2019 = df["Research and development tax credits -- Total deferred taxes"]["2019"]
total_deferred_tax_2018 = df["Research and development tax credits -- Total deferred taxes"]["2018"]
answer = total_deferred_tax_2019 - total_deferred_tax_2018