What was the percentage change in gross profit between 2018 and 2019?
{"": {"2019": "", "2018": "(dollars in thousands)", "2017": ""}, "Cost of revenue": {"2019": 149215, "2018": 144349, "2017": 142867}, "Gross profit": {"2019": 427308, "2018": 393542, "2017": 339118}, "Gross margin": {"2019": 74.1, "2018": 73.2, "2017": 70.4}}
gross_profit_2019 = df["Gross profit"]["2019"]
gross_profit_2018 = df["Gross profit"]["2018"]
answer = (gross_profit_2019 - gross_profit_2018) / gross_profit_2018 * 100.0