What was the change in the amount of long-lived assets between 2018 and 2019 in United States?
{"United States": {"2019": 239511, "2018": 115869}, "Asia": {"2019": 301020, "2018": 12274}, "Europe": {"2019": 59925, "2018": 59936}, "Total": {"2019": 600456, "2018": 188079}}
united_states_2019 = df["United States"]["2019"]
united_states_2018 = df["United States"]["2018"]
answer = united_states_2019 - united_states_2018