What is the percentage of the 2019 adjusted EBITDA over the sum of the adjusted EBITDA in 2018 and 2019? 
{"Operating costs": {"2019": -6942.0, "2018": -6946.0, "$ CHANGE": 4.0, "% CHANGE": 0.1}, "Adjusted EBITDA": {"2019": 5414.0, "2018": 5321.0, "$ CHANGE": 93.0, "% CHANGE": 1.7}, "Adjusted EBITDA margin": {"2019": 43.8, "2018": 43.4, "$ CHANGE": "", "% CHANGE": 0.4}}
adjusted_ebitda_2019 = df["Adjusted EBITDA"]["2019"]
adjusted_ebitda_2018 = df["Adjusted EBITDA"]["2018"]
answer = adjusted_ebitda_2019 / ( adjusted_ebitda_2019 + adjusted_ebitda_2018 ) * 100.0