What is the change in the revenue from 2018 to 2019?
{"Year ended 31 March": {"2019": "\u00a3m", "2018": "\u00a3m", "2017": "\u00a3m"}, "Revenue": {"2019": 23428, "2018": 23723, "2017": 24062}, "Operating costs a": {"2019": -16461, "2018": -16828, "2017": -17323}, "Depreciation and amortisation": {"2019": -3546, "2018": -3514, "2017": -3572}, "Operating profit": {"2019": 3421, "2018": 3381, "2017": 3167}, "Net finance expense": {"2019": -756, "2018": -764, "2017": -804}, "Associates and joint ventures": {"2019": 1, "2018": -1, "2017": -9}, "Profit before tax": {"2019": 2666, "2018": 2616, "2017": 2354}, "Tax": {"2019": -507, "2018": -584, "2017": -446}, "Profit for the period": {"2019": 2159, "2018": 2032, "2017": 1908}}
revenue_2019 = df["Revenue"]["2019"]
revenue_2018 = df["Revenue"]["2018"]
answer = revenue_2019 - revenue_2018