What was the change in the cost at 31st December in 2019 from 2018?
{"": {"2019": "\u00a3m", "2018": "\u00a3m"}, "Cost: -- At 1st January": {"2019": 445.8, "2018": 269.4}, "Cost: -- Share options issued to subsidiary company employees": {"2019": 2.2, "2018": 2.6}, "Cost: -- Additions": {"2019": 214.0, "2018": 173.8}, "Cost: -- At 31st December": {"2019": 662.0, "2018": 445.8}}
cost_2019 = df["Cost: -- At 31st December"]["2019"]
cost_2018 = df["Cost: -- At 1st January"]["2019"]
answer = cost_2019 - cost_2018