What is the % change in the foreign income from 2017 to 2018, in billions?
{"Domestic": {"2019": 3774, "2018": 3366, "2017": 3674}, "Foreign": {"2019": 8494, "2018": 9058, "2017": 8006}, "Income before provision for income taxes": {"2019": 12268, "2018": 12424, "2017": 11680}, "": {"2019": "", "2018": "", "2017": ""}}
foreign_income_2018 = df["Foreign"]["2018"]
foreign_income_2017 = df["Foreign"]["2017"]
answer = (foreign_income_2018 - foreign_income_2017) / foreign_income_2017 * 100.0