What is the increase / (decrease) in the External Operating Systems Software gross profit 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"}, "Systems -- External Systems Hardware gross profit": {"2019": 2622.0, "2018": 2590.0, "Yr.-to-Yr Percent/ Margin Change": 1.2}, "Systems -- External Systems Hardware gross profit margin": {"2019": 44.3, "2018": 40.7, "Yr.-to-Yr Percent/ Margin Change": 3.6}, "Systems -- External Operating Systems Software gross profit": {"2019": 1412.0, "2018": 1412.0, "Yr.-to-Yr Percent/ Margin Change": 0.0}, "Systems -- External Operating Systems Software gross profit margin": {"2019": 83.8, "2018": 84.5, "Yr.-to-Yr Percent/ Margin Change": -0.7}, "Systems -- External total gross profit": {"2019": 4034.0, "2018": 4002.0, "Yr.-to-Yr Percent/ Margin Change": 0.8}, "Systems -- External total gross profit margin": {"2019": 53.1, "2018": 49.8, "Yr.-to-Yr Percent/ Margin Change": 3.2}, "Systems -- Pre-tax income": {"2019": 701.0, "2018": 904.0, "Yr.-to-Yr Percent/ Margin Change": -22.4}, "Systems -- Pre-tax margin": {"2019": 8.4, "2018": 10.2, "Yr.-to-Yr Percent/ Margin Change": -1.8}}
external_os_software_gross_profit_2019 = df["Systems -- External Operating Systems Software gross profit"]["2019"]
external_os_software_gross_profit_2018 = df["Systems -- External Operating Systems Software gross profit"]["2018"]
answer = external_os_software_gross_profit_2019 - external_os_software_gross_profit_2018