What was the change in the amount of Investment in Sophos Limited in 2019 from 2018?
{"": {"31 March 2019": "$M", "31 March 2018": "$M"}, "Investment in Sophos Holdings Limited": {"31 March 2019": 1035.8, "31 March 2018": 1035.8}, "Investment in Sophos Limited": {"31 March 2019": 94.8, "31 March 2018": 63.3}, "At 31 March": {"31 March 2019": 1130.6, "31 March 2018": 1099.1}}
investment_2019 = df["Investment in Sophos Limited"]["31 March 2019"]
investment_2018 = df["Investment in Sophos Limited"]["31 March 2018"]
answer = investment_2019 - investment_2018