What was the change in U.S. assets between 2018 and 2019?
{"U.S.": {"April 26, 2019": 159, "April 27, 2018": 853}, "International": {"April 26, 2019": 3740, "April 27, 2018": 4538}, "Total": {"April 26, 2019": 3899, "April 27, 2018": 5391}}
us_assets_2019 = df["U.S."]["April 26, 2019"]
us_assets_2018 = df["U.S."]["April 27, 2018"]
answer = us_assets_2019 - us_assets_2018