What is the difference in total revenue between 2017 and 2018?
{"United States": {"2019": 799016, "2018": 880534, "2017": 944052}, "Americas excluding United States": {"2019": 94961, "2018": 101282, "2017": 116330}, "Europe, Middle East, and Africa": {"2019": 410485, "2018": 384196, "2017": 440135}, "Asia Pacific": {"2019": 130326, "2018": 145971, "2017": 115002}, "Total": {"2019": 1434788, "2018": 1511983, "2017": 1615519}}
total_2017 = df["Total"]["2017"]
total_2018 = df["Total"]["2018"]
answer = total_2017 - total_2018