What is the change in subscription solutions revenue between 2018 and 2019?
{"Revenues: -- Subscription solutions": {"2019": 642241.0, "2018": 464996.0, "2017": 310031.0}, "Revenues: -- Merchant solutions": {"2019": 935932.0, "2018": 608233.0, "2017": 363273.0}, "Revenues: -- ": {"2019": 1578173.0, "2018": 1073229.0, "2017": 673304.0}, "Percentage of revenues: -- Subscription solutions": {"2019": 40.7, "2018": 43.3, "2017": 46.0}, "Percentage of revenues: -- Merchant solutions": {"2019": 59.3, "2018": 56.7, "2017": 54.0}, "Percentage of revenues: -- Total revenues": {"2019": 100.0, "2018": 100.0, "2017": 100.0}}
revenue_2019 = df["Revenues: -- Subscription solutions"]["2019"]
revenue_2018 = df["Revenues: -- Subscription solutions"]["2018"]
answer = revenue_2019 - revenue_2018