What was the increase / (decrease) in the Pre-tax income from 2018 to 2019?
{"($ in millions) -- For the year ended December 31:": {"2019": 2019.0, "2018*": 2018.0, "Yr.-to-Yr. Percent/ Margin Change": "Yr.-to-Yr. Percent/ Margin Change"}, "Global Business Services -- External gross profit": {"2019": 4606.0, "2018*": 4448.0, "Yr.-to-Yr. Percent/ Margin Change": 3.5}, "Global Business Services -- External gross profit margin": {"2019": 27.7, "2018*": 26.8, "Yr.-to-Yr. Percent/ Margin Change": 0.9}, "Global Business Services -- Pre-tax income": {"2019": 1666.0, "2018*": 1629.0, "Yr.-to-Yr. Percent/ Margin Change": 2.2}, "Global Business Services -- Pre-tax margin": {"2019": 9.9, "2018*": 9.6, "Yr.-to-Yr. Percent/ Margin Change": 0.2}}
pretax_income_2019 = df["Global Business Services -- Pre-tax income"]["2019"]
pretax_income_2018 = df["Global Business Services -- Pre-tax income"]["2018*"]
answer = pretax_income_2019 - pretax_income_2018