What was the change in total backlog in 2019 from 2018?
{"": {"2019": "", "2018": "(in millions)"}, "Transportation Solutions": {"2019": 1639, "2018": 1779}, "Industrial Solutions": {"2019": 1315, "2018": 1245}, "Communications Solutions": {"2019": 361, "2018": 441}, "Total": {"2019": 3315, "2018": 3465}}
total_2019 = df["Total"]["2019"]
total_2018 = df["Total"]["2018"]
answer = total_2019 - total_2018