What is the increase/ (decrease) in External Systems Hardware gross profit 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"}, "Systems -- External Systems Hardware gross profit": {"2018": 2590.0, "2017": 2893.0, "Yr.-to-Yr. Percent/ Margin Change": -10.5}, "Systems -- External Systems Hardware gross profit margin": {"2018": 40.7, "2017": 44.6, "Yr.-to-Yr. Percent/ Margin Change": -3.8}, "Systems -- External Operating Systems Software gross profit": {"2018": 1412.0, "2017": 1469.0, "Yr.-to-Yr. Percent/ Margin Change": -3.9}, "Systems -- External Operating Systems Software gross profit margin": {"2018": 84.5, "2017": 86.4, "Yr.-to-Yr. Percent/ Margin Change": -1.9}, "Systems -- External total gross profit": {"2018": 4002.0, "2017": 4362.0, "Yr.-to-Yr. Percent/ Margin Change": -8.2}, "Systems -- External total gross profit margin": {"2018": 49.8, "2017": 53.2, "Yr.-to-Yr. Percent/ Margin Change": -3.4}, "Systems -- Pre-tax income": {"2018": 904.0, "2017": 1128.0, "Yr.-to-Yr. Percent/ Margin Change": -19.9}, "Systems -- Pre-tax margin": {"2018": 10.2, "2017": 12.6, "Yr.-to-Yr. Percent/ Margin Change": -2.4}}
external_systems_hardware_gross_profit_2018 = df["Systems -- External Systems Hardware gross profit"]["2018"]
external_systems_hardware_gross_profit_2017 = df["Systems -- External Systems Hardware gross profit"]["2017"]
answer = external_systems_hardware_gross_profit_2018 - external_systems_hardware_gross_profit_2017