What was the change in the net sales from United States between 2017 and 2018?
{"United States": {"2019": 279904, "2018": 313489, "2017": 287092}, "Singapore": {"2019": 32957, "2018": 6724, "2017": 5596}, "Taiwan": {"2019": 19810, "2018": 20802, "2017": 18586}, "China": {"2019": 87342, "2018": 79380, "2017": 66510}, "Czech Republic": {"2019": 33214, "2018": 36528, "2017": 34476}, "Other non-U.S.": {"2019": 15772, "2018": 13560, "2017": 10733}, "Consolidated net sales": {"2019": 468999, "2018": 470483, "2017": 422993}}
united_states_2018 = df["United States"]["2018"]
united_states_2017 = df["United States"]["2017"]
answer = united_states_2018 - united_states_2017