What is the change in gross debt in 2019 from 2018?
{"Gross Debt": {"2019": 35039.0, "2018": 36352.0, "2017(1)": 38053.0}, "Cash and cash equivalents": {"2019": -1690.0, "2018": -488.0, "2017(1)": -551.0}, "Net debt": {"2019": 33349.0, "2018": 35864.0, "2017(1)": 37502.0}, "Adjusted EBITDA excluding integration and transformation costs and special items (1)": {"2019": 9070.0, "2018": 9040.0, "2017(1)": 8686.0}, "Net Debt to Adjusted EBITDA Ratio": {"2019": 3.7, "2018": 4.0, "2017(1)": 4.3}}
gross_debt_2019 = df["Gross Debt"]["2019"]
gross_debt_2018 = df["Gross Debt"]["2018"]
answer = gross_debt_2019 - gross_debt_2018