What was the change in the percentage of total revenue in U.S. between 2018 and 2019?
{"U.S.": {"2019": 55, "2018": 51, "2017": 55}, "Asia": {"2019": 16, "2018": 21, "2017": 17}, "Africa": {"2019": 8, "2018": 7, "2017": 7}, "Europe": {"2019": 2, "2018": 2, "2017": 2}, "Latin America": {"2019": 18, "2018": 17, "2017": 18}}
us_2019 = df["U.S."]["2019"]
us_2018 = df["U.S."]["2018"]
answer = us_2019 - us_2018