What was the change in free cash flow between 2018 and 2019?
{"Reconciliation of free cash flow: -- Net cash provided by operating activities": {"2019": 115549.0, "2018": 90253.0, "2017": 67510.0}, "Reconciliation of free cash flow: -- Capital expenditures": {"2019": -18034.0, "2018": -14895.0, "2017": -7100.0}, "Reconciliation of free cash flow: -- Capitalized software costs": {"2019": -24668.0, "2018": -25515.0, "2017": -20571.0}, "Reconciliation of free cash flow: -- Free cash flow": {"2019": 72847.0, "2018": 49843.0, "2017": 39839.0}, "Reconciliation of free cash flow: -- Free cash flow margin": {"2019": 12.6, "2018": 9.3, "2017": 8.3}}
free_cash_flow_2019 = df["Reconciliation of free cash flow: -- Free cash flow"]["2019"]
free_cash_flow_2018 = df["Reconciliation of free cash flow: -- Free cash flow"]["2018"]
answer = free_cash_flow_2019 - free_cash_flow_2018