What is the change in the net debt in 2019?
{"Debt due within one year": {"2019": 3881.0, "2018": 4645.0, "$ CHANGE": -764.0, "% CHANGE": -16.4}, "Long-term debt": {"2019": 22415.0, "2018": 19760.0, "$ CHANGE": 2655.0, "% CHANGE": 13.4}, "Preferred shares(1)": {"2019": 2002, "2018": 2002, "$ CHANGE": "\u2013", "% CHANGE": "\u2013"}, "Cash and cash equivalents": {"2019": -145.0, "2018": -425.0, "$ CHANGE": 280.0, "% CHANGE": 65.9}, "Net debt": {"2019": 28153.0, "2018": 25982.0, "$ CHANGE": 2171.0, "% CHANGE": 8.4}}
net_debt_2019 = df["Net debt"]["2019"]
net_debt_2018 = df["Net debt"]["2018"]
answer = net_debt_2019 - net_debt_2018