What is the increase / (decrease) in the internal revenue from 2018 to 2019?
{"($ in millions) -- For the year ended December 31:": {"2019": 2019, "2018": 2018, "Yr.-to-Yr. Percent Change": "Yr.-to-Yr. Percent Change"}, "($ in millions) -- External revenue": {"2019": 1400.0, "2018": 1590.0, "Yr.-to-Yr. Percent Change": -11.9}, "($ in millions) -- Internal revenue": {"2019": 1232.0, "2018": 1610.0, "Yr.-to-Yr. Percent Change": -23.5}, "($ in millions) -- Total revenue": {"2019": 2632.0, "2018": 3200.0, "Yr.-to-Yr. Percent Change": -17.8}, "($ in millions) -- Pre-tax income": {"2019": 1055.0, "2018": 1361.0, "Yr.-to-Yr. Percent Change": -22.5}}
internal_revenue_2019 = df["($ in millions) -- Internal revenue"]["2019"]
internal_revenue_2018 = df["($ in millions) -- Internal revenue"]["2018"]
answer = internal_revenue_2019 - internal_revenue_2018