What was the change in working capital between 2015 and 2016?
{"Balance sheet and other data: -- Cash and cash equivalents": {"2019": 119629, "2018": 146061, "2017": 96329, "2016": 140634, "2015": 128358}, "Balance sheet and other data: -- Working capital": {"2019": 167879, "2018": 152793, "2017": 119433, "2016": 150485, "2015": 131971}, "Balance sheet and other data: -- Total assets": {"2019": 557799, "2018": 440985, "2017": 371641, "2016": 261245, "2015": 226095}, "Balance sheet and other data: -- Total long-term obligations": {"2019": 115143, "2018": 88126, "2017": 94311, "2016": 30297, "2015": 26885}, "Balance sheet and other data: -- Total stockholders' equity": {"2019": 355651, "2018": 277589, "2017": 232827, "2016": 191249, "2015": 170131}}
working_capital_2016 = df["Balance sheet and other data: -- Working capital"]["2016"]
working_capital_2015 = df["Balance sheet and other data: -- Working capital"]["2015"]
answer = working_capital_2016 - working_capital_2015