What is the change in the adjusted EBITDA growth % from 2018 to 2019?
{"Bookings": {"2019": 1002320.0, "2018": 2779782.0, "2017": 1234013.0}, "Backlog": {"2019": 3400952.0, "2018": 4064451.0, "2017": 2536499.0}, "Sales": {"2019": 1496475.0, "2018": 1202898.0, "2017": 1107709.0}, "Sales growth %": {"2019": 24.0, "2018": 9.0, "2017": ""}, "Research & development": {"2019": 50132.0, "2018": 52398.0, "2017": 52652.0}, "Operating income": {"2019": 86237.0, "2018": 24382.0, "2017": 2628.0}, "Net income (loss) per share, continuing operations attributable to Cubic": {"2019": 1.67, "2018": 0.29, "2017": -0.95}, "Adjusted EBITDA (1)": {"2019": 146594.0, "2018": 104561.0, "2017": 87470.0}, "Adjusted EBITDA growth %": {"2019": 40.0, "2018": 20.0, "2017": ""}, "Adjusted earnings per share (1)": {"2019": 3.13, "2018": 2.19, "2017": 1.62}, "Adjusted earnings per share growth %": {"2019": 43.0, "2018": 35.0, "2017": ""}, "Cash dividend per share": {"2019": 0.27, "2018": 0.27, "2017": 0.27}, "Long-term debt, inclusive of current portion": {"2019": 199824.0, "2018": 199793.0, "2017": 199761.0}}
adjusted_ebitda_growth_2019 = df["Adjusted EBITDA growth %"]["2019"]
adjusted_ebitda_growth_2018 = df["Adjusted EBITDA growth %"]["2018"]
answer = adjusted_ebitda_growth_2019 - adjusted_ebitda_growth_2018