What is the increase/ (decrease) in Pre-tax income from 2017 to 2018
{"($ in millions) -- For the year ended December 31:": {"2018*": 2018.0, "2017*": 2017.0, "Yr.-to-Yr. Percent/ Margin Change": "Yr.-to-Yr. Percent/ Margin Change"}, "Global Technology Services -- External total gross profit": {"2018*": 10035.0, "2017*": 10022.0, "Yr.-to-Yr. Percent/ Margin Change": 0.1}, "Global Technology Services -- External total gross profit margin": {"2018*": 34.4, "2017*": 34.3, "Yr.-to-Yr. Percent/ Margin Change": 0.1}, "Global Technology Services -- Pre-tax income": {"2018*": 1781.0, "2017*": 2618.0, "Yr.-to-Yr. Percent/ Margin Change": -32.0}, "Global Technology Services -- Pre-tax margin": {"2018*": 5.9, "2017*": 8.8, "Yr.-to-Yr. Percent/ Margin Change": -2.8}}
pretax_income_2018 = df["Global Technology Services -- Pre-tax income"]["2018*"]
pretax_income_2017 = df["Global Technology Services -- Pre-tax income"]["2017*"]
answer = pretax_income_2018 - pretax_income_2017