What was the change in day sales outstanding between 2018 and 2019?
{"(in days) -- Three Months Ended": {"April 26, 2019": 26, "April 27, 2018": 27, "April 28, 2017": 28}, "(in days) -- Days sales outstanding (1)": {"April 26, 2019": 70, "April 27, 2018": 58, "April 28, 2017": 45}, "(in days) -- Days inventory outstanding (2)": {"April 26, 2019": 21, "April 27, 2018": 18, "April 28, 2017": 26}, "(in days) -- Days payables outstanding (3)": {"April 26, 2019": -87, "April 27, 2018": -90, "April 28, 2017": -56}, "(in days) -- Cash conversion cycle (4)": {"April 26, 2019": 3, "April 27, 2018": -14, "April 28, 2017": 15}}
days_sales_2019 = df["(in days) -- Days sales outstanding (1)"]["April 26, 2019"]
days_sales_2018 = df["(in days) -- Days sales outstanding (1)"]["April 27, 2018"]
answer = days_sales_2019 - days_sales_2018