What was the percentage change in service revenue between 2018 and 2019?
{"Service": {"July 27, 2019": 11709, "July 28, 2018": 11431, "Increase (Decrease)": 278}, "Product": {"July 27, 2019": 6758, "July 28, 2018": 8254, "Increase (Decrease)": -1496}, "Total": {"July 27, 2019": 18467, "July 28, 2018": 19685, "Increase (Decrease)": -1218}, "Reported as: -- Current": {"July 27, 2019": 10668, "July 28, 2018": 11490, "Increase (Decrease)": -822}, "Reported as: -- Noncurrent": {"July 27, 2019": 7799, "July 28, 2018": 8195, "Increase (Decrease)": -396}, "Reported as: -- Total": {"July 27, 2019": 18467, "July 28, 2018": 19685, "Increase (Decrease)": -1218}}
service_2019 = df["Service"]["July 27, 2019"]
service_2018 = df["Service"]["July 28, 2018"]
answer = (service_2019 - service_2018) / service_2018 * 100.0