What was the increase / (decrease) in the Pre-tax margin 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 Technology Services -- External total gross profit": {"2019": 9515.0, "2018*": 10035.0, "Yr.-to-Yr. Percent/ Margin Change": -5.2}, "Global Technology Services -- External total gross profit margin": {"2019": 34.8, "2018*": 34.4, "Yr.-to-Yr. Percent/ Margin Change": 0.3}, "Global Technology Services -- Pre-tax income": {"2019": 1645.0, "2018*": 1781.0, "Yr.-to-Yr. Percent/ Margin Change": -7.6}, "Global Technology Services -- Pre-tax margin": {"2019": 5.8, "2018*": 5.9, "Yr.-to-Yr. Percent/ Margin Change": -0.2}}
pretax_margin_2019 = df["Global Technology Services -- Pre-tax margin"]["2019"]
pretax_margin_2018 = df["Global Technology Services -- Pre-tax margin"]["2018*"]
answer = pretax_margin_2019 - pretax_margin_2018