What was the change in the net sales from U.S. from 2018 to 2019?
{"Net Sales: -- U.S.": {"April 27, 2019": 540.5, "April 28, 2018": 487.5, "April 29, 2017": 506.9}, "Net Sales: -- Malta": {"April 27, 2019": 148.5, "April 28, 2018": 184.0, "April 29, 2017": 155.5}, "Net Sales: -- China": {"April 27, 2019": 113.7, "April 28, 2018": 117.3, "April 29, 2017": 127.7}, "Net Sales: -- Canada": {"April 27, 2019": 101.6, "April 28, 2018": 54.4, "April 29, 2017": "\u2014"}, "Net Sales: -- Other": {"April 27, 2019": 96.0, "April 28, 2018": 65.1, "April 29, 2017": 26.4}, "Net Sales: -- Total Net Sales": {"April 27, 2019": 1000.3, "April 28, 2018": 908.3, "April 29, 2017": 816.5}, "Property, Plant and Equipment, Net: -- U.S.": {"April 27, 2019": 83.9, "April 28, 2018": 63.3, "April 29, 2017": 44.9}, "Property, Plant and Equipment, Net: -- Malta": {"April 27, 2019": 33.0, "April 28, 2018": 36.8, "April 29, 2017": 26.4}, "Property, Plant and Equipment, Net: -- Belgium": {"April 27, 2019": 22.1, "April 28, 2018": 25.0, "April 29, 2017": "\u2014"}, "Property, Plant and Equipment, Net: -- China": {"April 27, 2019": 18.6, "April 28, 2018": 7.2, "April 29, 2017": 5.9}, "Property, Plant and Equipment, Net: -- Other": {"April 27, 2019": 34.3, "April 28, 2018": 29.9, "April 29, 2017": 13.4}, "Property, Plant and Equipment, Net: -- Total Property, Plant and Equipment, Net": {"April 27, 2019": 191.9, "April 28, 2018": 162.2, "April 29, 2017": 90.6}}
net_sales_2019 = df["Net Sales: -- U.S."]["April 27, 2019"]
net_sales_2018 = df["Net Sales: -- U.S."]["April 28, 2018"]
answer = net_sales_2019 - net_sales_2018