What is the percentage change in net debt in 2019 from 2018?
{"": {"2019": "\u00a3m", "2018": "\u00a3m"}, "Total equity": {"2019": 826.3, "2018": 766.9}, "Net debt": {"2019": 334.1, "2018": 235.8}, "Total invested capital": {"2019": 1160.4, "2018": 1002.7}, "Average invested capital": {"2019": 1081.6, "2018": 992.9}, "Average invested capital (excluding IFRS 16)": {"2019": 1061.2, "2018": 992.9}, "Operating profit as reported under IFRS": {"2019": 245.0, "2018": 299.1}, "Adjustments (see adjusted operating profit)": {"2019": 37.7, "2018": -34.2}, "Adjusted operating profit": {"2019": 282.7, "2018": 264.9}, "Taxation": {"2019": -80.6, "2018": -73.1}, "Adjusted operating profit after tax": {"2019": 202.1, "2018": 191.8}, "Adjusted operating profit after tax (excluding IFRS 16)": {"2019": 201.2, "2018": 191.8}, "Return in invested capital": {"2019": 18.7, "2018": 19.3}, "Return in invested capital (excluding IFRS 16)": {"2019": 19.0, "2018": 19.3}}
net_debt_2019 = df["Net debt"]["2019"]
net_debt_2018 = df["Net debt"]["2018"]
answer = (net_debt_2019 - net_debt_2018) / net_debt_2018 * 100.0