What was the change in deferred tax asset in 2019 from 2018?
{"": {"2019": "\u00a3m", "2018": "\u00a3m"}, "Amounts owed by Group undertakings": {"2019": 414.7, "2018": 439.9}, "Deferred tax asset": {"2019": 1.2, "2018": 0.8}, "Total": {"2019": 415.9, "2018": 440.7}}
deferred_tax_asset_2019 = df["Deferred tax asset"]["2019"]
deferred_tax_asset_2018 = df["Deferred tax asset"]["2018"]
answer = deferred_tax_asset_2019 - deferred_tax_asset_2018