What is the total Adjusted EBITDA for Telematics Systems and Software & Subscription Services in 2018?
{"": {"2019": "Segment", "2018": "", "$ Change": "", "% Change": ""}, "Telematics Systems": {"2019": 40821.0, "2018": 48943.0, "$ Change": -8122.0, "% Change": -17.0}, "Software & Subscription Services": {"2019": 13093.0, "2018": 8233.0, "$ Change": 4860.0, "% Change": 59.0}, "Corporate Expense": {"2019": -5699.0, "2018": -4794.0, "$ Change": -905.0, "% Change": 19.0}, "Total Adjusted EBITDA": {"2019": 48215.0, "2018": 52382.0, "$ Change": -4167.0, "% Change": -8.0}}
telematics_systems = df["Telematics Systems"]["2018"]
software_subscription_services = df["Software & Subscription Services"]["2018"]
answer = telematics_systems + software_subscription_services