What is the change in goodwill from 2016 to 2017?
{"": {"2019": "", "2018": "", "2017": "(Dollars in millions)", "2016": "", "2015": ""}, "Net property, plant and equipment(1)": {"2019": 26079, "2018": 26408, "2017": 26852, "2016": 17039, "2015": 18069}, "Goodwill(1)(2)": {"2019": 21534, "2018": 28031, "2017": 30475, "2016": 19650, "2015": 20742}, "Total assets(3)(4)": {"2019": 64742, "2018": 70256, "2017": 75611, "2016": 47017, "2015": 47604}, "Total long-term debt(3)(5)": {"2019": 34694, "2018": 36061, "2017": 37726, "2016": 19993, "2015": 20225}, "Total stockholders equity": {"2019": 13470, "2018": 19828, "2017": 23491, "2016": 13399, "2015": 14060}}
goodwill12_2017 = df["Goodwill(1)(2)"]["2017"]
goodwill12_2016 = df["Goodwill(1)(2)"]["2016"]
answer = goodwill12_2017 - goodwill12_2016