What was the change in short-term investments between 2018 and 2019?
{"Cash and cash equivalents": {"April 26, 2019": 2325, "April 27, 2018": 2941}, "Short-term investments": {"April 26, 2019": 1574, "April 27, 2018": 2450}, "Total": {"April 26, 2019": 3899, "April 27, 2018": 5391}}
shortterm_investments_april_26_2019 = df["Short-term investments"]["April 26, 2019"]
shortterm_investments_april_27_2018 = df["Short-term investments"]["April 27, 2018"]
answer = shortterm_investments_april_26_2019 - shortterm_investments_april_27_2018