What is the increase / (decrease) in the Pre-tax margin 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 Business Services -- External gross profit": {"2018*": 4448.0, "2017*": 4033.0, "Yr.-to-Yr. Percent/ Margin Change": 10.3}, "Global Business Services -- External gross profit margin": {"2018*": 26.8, "2017*": 25.1, "Yr.-to-Yr. Percent/ Margin Change": 1.7}, "Global Business Services -- Pre-tax income": {"2018*": 1629.0, "2017*": 1303.0, "Yr.-to-Yr. Percent/ Margin Change": 25.0}, "Global Business Services -- Pre-tax margin": {"2018*": 9.6, "2017*": 7.9, "Yr.-to-Yr. Percent/ Margin Change": 1.7}}
pretax_margin_2018 = df["Global Business Services -- Pre-tax margin"]["2018*"]
pretax_margin_2017 = df["Global Business Services -- Pre-tax margin"]["2017*"]
answer = pretax_margin_2018 - pretax_margin_2017