What is the increase/ (decrease) in Internal Revenue from 2017 to 2018
{"($ in millions) -- For the year ended December 31:": {"2018": 2018, "2017": 2017, "Yr.-to-Yr. Percent Change": "Yr.-to-Yr. Percent Change"}, "Results of Operations -- External revenue": {"2018": 1590.0, "2017": 1696.0, "Yr.-to-Yr. Percent Change": -6.3}, "Results of Operations -- Internal revenue": {"2018": 1610.0, "2017": 1471.0, "Yr.-to-Yr. Percent Change": 9.5}, "Results of Operations -- Total revenue": {"2018": 3200.0, "2017": 3168.0, "Yr.-to-Yr. Percent Change": 1.0}, "Results of Operations -- Pre-tax income": {"2018": 1361.0, "2017": 1278.0, "Yr.-to-Yr. Percent Change": 6.5}}
internal_revenue_2018 = df["Results of Operations -- Internal revenue"]["2018"]
internal_revenue_2017 = df["Results of Operations -- Internal revenue"]["2017"]
answer = internal_revenue_2018 - internal_revenue_2017