What is the increase / (decrease) in the external 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"}, "Cloud & Cognitive Software -- External gross profit": {"2018*": 17224.0, "2017*": 16986.0, "Yr.-to-Yr. Percent/ Margin Change": 1.4}, "Cloud & Cognitive Software -- External gross profit margin": {"2018*": 77.6, "2017*": 78.1, "Yr.-to-Yr. Percent/ Margin Change": -0.5}, "Cloud & Cognitive Software -- Pre-tax income": {"2018*": 8882.0, "2017*": 8068.0, "Yr.-to-Yr. Percent/ Margin Change": 10.1}, "Cloud & Cognitive Software -- Pre-tax margin": {"2018*": 35.0, "2017*": 32.4, "Yr.-to-Yr. Percent/ Margin Change": 2.6}}
external_gross_profit_2018 = df["Cloud & Cognitive Software -- External gross profit"]["2018*"]
external_gross_profit_2017 = df["Cloud & Cognitive Software -- External gross profit"]["2017*"]
answer = external_gross_profit_2018 - external_gross_profit_2017