What is the change between 2018 and 2019 year ended's monthly recurring revenue ?
{"": {"2019": "(in thousands)", "2018": ""}, "Monthly Recurring Revenue": {"2019": 53898, "2018": 40932}, "Gross Merchandise Volume": {"2019": 61138457, "2018": 41103238}}
monthly_recurring_revenue_2019 = df["Monthly Recurring Revenue"]["2019"]
monthly_recurring_revenue_2018 = df["Monthly Recurring Revenue"]["2018"]
answer = monthly_recurring_revenue_2019 - monthly_recurring_revenue_2018