What is the increase / (decrease) in Operating Systems Software from 2018 to 2019?
{"($ in millions) -- For the year ended December 31:": {"2019": 2019, "2018": 2018, "Yr.-to-Yr. Percent Change": "Yr.-to-Yr. Percent Change", "Yr.-to-Yr. Percent Change Adjusted for Currency": "Yr.-to-Yr. Percent Change Adjusted for Currency"}, "($ in millions) -- Systems external revenue": {"2019": 7604.0, "2018": 8034.0, "Yr.-to-Yr. Percent Change": -5.3, "Yr.-to-Yr. Percent Change Adjusted for Currency": -4.1}, "($ in millions) -- Systems Hardware": {"2019": 5918.0, "2018": 6363.0, "Yr.-to-Yr. Percent Change": -7.0, "Yr.-to-Yr. Percent Change Adjusted for Currency": -5.9}, "($ in millions) -- IBM Z": {"2019": "", "2018": "", "Yr.-to-Yr. Percent Change": -1.1, "Yr.-to-Yr. Percent Change Adjusted for Currency": -0.3}, "($ in millions) -- Power Systems": {"2019": "", "2018": "", "Yr.-to-Yr. Percent Change": -13.5, "Yr.-to-Yr. Percent Change Adjusted for Currency": -12.1}, "($ in millions) -- Storage Systems": {"2019": "", "2018": "", "Yr.-to-Yr. Percent Change": -8.9, "Yr.-to-Yr. Percent Change Adjusted for Currency": -7.6}, "($ in millions) -- Operating Systems Software": {"2019": 1686.0, "2018": 1671.0, "Yr.-to-Yr. Percent Change": 0.9, "Yr.-to-Yr. Percent Change Adjusted for Currency": 2.6}}
operating_systems_software_2019 = df["($ in millions) -- Operating Systems Software"]["2019"]
operating_systems_software_2018 = df["($ in millions) -- Operating Systems Software"]["2018"]
answer = operating_systems_software_2019 - operating_systems_software_2018