What was the change in the Average invested capital less average impairment in 2019 from 2018?
{"EBIT less Tax": {"2019": 205.1, "2018": 1.2, "2017": 38.8}, "Impairment reversal": {"2019": -120.0, "2018": "-", "2017": "-"}, "EBIT less tax and impairment": {"2019": 85.1, "2018": 1.2, "2017": 38.8}, "Average invested capital": {"2019": 1627.7, "2018": 1437.7, "2017": 1396.9}, "Average impairment ": {"2019": 98.2, "2018": 185.0, "2017": 185.0}, "Average invested capital less average impairment": {"2019": 1725.9, "2018": 1622.7, "2017": 1581.9}, "Adjusted RoIC": {"2019": 4.9, "2018": 0.1, "2017": 2.4}}
average_invested_capital_2019 = df["Average invested capital less average impairment"]["2019"]
average_invested_capital_2018 = df["Average invested capital less average impairment"]["2018"]
answer = average_invested_capital_2019 - average_invested_capital_2018