What is the total gross debt over the three years?
{"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}}
total_gross_debt = df["Gross Debt"]["2019"] + df["Gross Debt"]["2018"] + df["Gross Debt"]["2017(1)"]
answer = total_gross_debt