What was the change in the earnings from U.S. between 2017 and 2018?
{"U.S.": {"2019": 15103, "2018": 30815, "2017": 9315}, "Non-U.S.": {"2019": 35163, "2018": 27288, "2017": 30938}, "Total": {"2019": 50266, "2018": 58103, "2017": 40253}}
us_2018 = df["U.S."]["2018"]
us_2017 = df["U.S."]["2017"]
answer = us_2018 - us_2017