What was the change in net property and equipment from United States between 2017 and 2018?
{"Property and equipment, net: -- United States": {"July 27, 2019": 2266, "July 28, 2018": 2487, "July 29, 2017": 2711}, "Property and equipment, net: -- International": {"July 27, 2019": 523, "July 28, 2018": 519, "July 29, 2017": 611}, "Property and equipment, net: -- Total": {"July 27, 2019": 2789, "July 28, 2018": 3006, "July 29, 2017": 3322}}
net_property_2018 = df["Property and equipment, net: -- United States"]["July 28, 2018"]
net_property_2017 = df["Property and equipment, net: -- United States"]["July 29, 2017"]
answer = net_property_2018 - net_property_2017