What was the change in working capital between 2015 and 2016?
{"": {"April 26, 2019": "", "April 27, 2018 (2)": "", "April 28, 2017 (2)": "", "April 29, 2016": "(In millions)", "April 24, 2015": ""}, "Cash, cash equivalents and short-term investments": {"April 26, 2019": 3899, "April 27, 2018 (2)": 5391, "April 28, 2017 (2)": 4921, "April 29, 2016": 5303, "April 24, 2015": 5326}, "Working capital": {"April 26, 2019": 1743, "April 27, 2018 (2)": 3421, "April 28, 2017 (2)": 2178, "April 29, 2016": 2786, "April 24, 2015": 4064}, "Total assets": {"April 26, 2019": 8741, "April 27, 2018 (2)": 9991, "April 28, 2017 (2)": 9562, "April 29, 2016": 10037, "April 24, 2015": 9401}, "Total debt": {"April 26, 2019": 1793, "April 27, 2018 (2)": 1926, "April 28, 2017 (2)": 1993, "April 29, 2016": 2339, "April 24, 2015": 1487}, "Total deferred revenue and financed unearned services revenue": {"April 26, 2019": 3668, "April 27, 2018 (2)": 3363, "April 28, 2017 (2)": 3213, "April 29, 2016": 3385, "April 24, 2015": 3197}, "Total stockholders' equity": {"April 26, 2019": 1090, "April 27, 2018 (2)": 2276, "April 28, 2017 (2)": 2949, "April 29, 2016": 2881, "April 24, 2015": 3414}}
working_capital_april_29_2016 = df["Working capital"]["April 29, 2016"]
working_capital_april_24_2015 = df["Working capital"]["April 24, 2015"]
answer = working_capital_april_29_2016 - working_capital_april_24_2015