What was the increase / (decrease) in the Pre-tax income from 2018 to 2019?
{"($ in millions) -- For thee year ended December 31:": {"2019": 2019.0, "2018*": 2018.0, "Yr.-to-Yr. Percent/ Margin Change**": "Yr.-to-Yr. Percent/ Margin Change**"}, "Cloud & Cognitive Software -- External gross profit": {"2019": 17790.0, "2018*": 17224.0, "Yr.-to-Yr. Percent/ Margin Change**": 3.3}, "Cloud & Cognitive Software -- External gross profit margin": {"2019": 76.7, "2018*": 77.6, "Yr.-to-Yr. Percent/ Margin Change**": -0.9}, "Cloud & Cognitive Software -- Pre-tax income": {"2019": 7952.0, "2018*": 8882.0, "Yr.-to-Yr. Percent/ Margin Change**": -10.5}, "Cloud & Cognitive Software -- Pre-tax margin": {"2019": 30.6, "2018*": 35.0, "Yr.-to-Yr. Percent/ Margin Change**": -4.4}}
pretax_income_2019 = df["Cloud & Cognitive Software -- Pre-tax income"]["2019"]
pretax_income_2018 = df["Cloud & Cognitive Software -- Pre-tax income"]["2018*"]
answer = pretax_income_2019 - pretax_income_2018