What is the increase/ (decrease) in Used equipment sales from 2018 to 2019
{"($ in millions) -- For the year ended December 31:": {"2019": 2019, "2018": 2018, "2017": 2017}, "Cloud & Cognitive Software* -- Software": {"2019": 18712, "2018": 17970, "2017": 17681}, "Cloud & Cognitive Software* -- Services": {"2019": 4321, "2018": 4082, "2017": 3920}, "Cloud & Cognitive Software* -- Systems": {"2019": 166, "2018": 156, "2017": 150}, "Global Business Services* -- Services": {"2019": 16363, "2018": 16238, "2017": 15728}, "Global Business Services* -- Software": {"2019": 156, "2018": 151, "2017": 179}, "Global Business Services* -- Systems": {"2019": 115, "2018": 206, "2017": 165}, "Global Technology Services* -- Services": {"2019": 20768, "2018": 22222, "2017": 21913}, "Global Technology Services* -- Maintenance": {"2019": 5183, "2018": 5484, "2017": 5783}, "Global Technology Services* -- Systems": {"2019": 1072, "2018": 1069, "2017": 1207}, "Global Technology Services* -- Software": {"2019": 338, "2018": 371, "2017": 310}, "Systems -- Servers": {"2019": 3746, "2018": 3996, "2017": 3993}, "Systems -- Storage": {"2019": 1920, "2018": 2114, "2017": 2243}, "Systems -- Software": {"2019": 1528, "2018": 1499, "2017": 1520}, "Systems -- Services": {"2019": 410, "2018": 425, "2017": 438}, "Global Financing -- Financing": {"2019": 1120, "2018": 1223, "2017": 1167}, "Global Financing -- Used equipment sales": {"2019": 281, "2018": 366, "2017": 530}}
used_equipment_sales_2019 = df["Global Financing -- Used equipment sales"]["2019"]
used_equipment_sales_2018 = df["Global Financing -- Used equipment sales"]["2018"]
answer = used_equipment_sales_2019 - used_equipment_sales_2018