What was the change in Revenue in 2019 from 2018?
{"": {"Year-ended 31 March 2019": "$M", "Year-ended 31 March 2018 Restated See note 2": "$M"}, "Revenue": {"Year-ended 31 March 2019": 710.6, "Year-ended 31 March 2018 Restated See note 2": 639.0}, "Net deferral of revenue (see note 23)": {"Year-ended 31 March 2019": 49.7, "Year-ended 31 March 2018 Restated See note 2": 129.6}, "Billings": {"Year-ended 31 March 2019": 760.3, "Year-ended 31 March 2018 Restated See note 2": 768.6}, "Currency revaluation": {"Year-ended 31 March 2019": 25.9, "Year-ended 31 March 2018 Restated See note 2": 18.7}, "Constant currency billings": {"Year-ended 31 March 2019": 786.2, "Year-ended 31 March 2018 Restated See note 2": 787.3}}
revenue_2019 = df["Revenue"]["Year-ended 31 March 2019"]
revenue_2018 = df["Revenue"]["Year-ended 31 March 2018 Restated See note 2"]
answer = revenue_2019 - revenue_2018