What is the change in the Revenue constant currency growth rate from 2018 to 2019?
{"": {"2019": "", "2018": "(dollars in thousands) ", "2017": ""}, "Revenue constant currency growth rate (1)": {"2019": 32, "2018": 38, "2017": 39}, "Revenue retention rate": {"2019": 111, "2018": 110, "2017": 111}, "Total customers (2)": {"2019": 34400, "2018": 30400, "2017": 26400}, "Gross profit percentage  ": {"2019": 73, "2018": 73, "2017": 73}, "Adjusted EBITDA (1)": {"2019": 54008, "2018": 25752, "2017": 12457}}
revenue_constant_currency_growth_rate_1_2019 = df["Revenue constant currency growth rate (1)"]["2019"]
revenue_constant_currency_growth_rate_1_2018 = df["Revenue constant currency growth rate (1)"]["2018"]
answer = revenue_constant_currency_growth_rate_1_2019 - revenue_constant_currency_growth_rate_1_2018