What was the percentage change in the total gross margin from all segments between 2018 and 2019?
{"Revenue: -- Americas  .": {"July 27, 2019": 30927, "July 28, 2018": 29070, "July 29, 2017": 28351}, "Revenue: -- EMEA  .": {"July 27, 2019": 13100, "July 28, 2018": 12425, "July 29, 2017": 12004}, "Revenue: -- APJC .": {"July 27, 2019": 7877, "July 28, 2018": 7834, "July 29, 2017": 7650}, "Revenue: -- Total": {"July 27, 2019": 51904, "July 28, 2018": 49330, "July 29, 2017": 48005}, "Gross margin: -- Americas .": {"July 27, 2019": 20338, "July 28, 2018": 18792, "July 29, 2017": 18284}, "Gross margin: -- EMEA  .": {"July 27, 2019": 8457, "July 28, 2018": 7945, "July 29, 2017": 7855}, "Gross margin: -- APJC .": {"July 27, 2019": 4683, "July 28, 2018": 4726, "July 29, 2017": 4741}, "Gross margin: -- Segment total .": {"July 27, 2019": 33479, "July 28, 2018": 31463, "July 29, 2017": 30880}, "Gross margin: -- Unallocated corporate items .": {"July 27, 2019": -813, "July 28, 2018": -857, "July 29, 2017": -656}, "Gross margin: -- Total": {"July 27, 2019": 32666, "July 28, 2018": 30606, "July 29, 2017": 30224}}
total_gross_margin_2019 = df["Gross margin: -- Total"]["July 27, 2019"]
total_gross_margin_2018 = df["Gross margin: -- Total"]["July 28, 2018"]
answer = (total_gross_margin_2019 - total_gross_margin_2018) / total_gross_margin_2018 * 100.0