How much did Subscription service revenue increase from the year 2018 to 2019?
{"Subscription service revenue": {"2019": 849267, "2018": 681138, "2017": 489274}, "Subscription software revenue": {"2019": 25739, "2018": 23262, "2017": 17081}, "Hardware and services": {"2019": 13184, "2018": 12594, "2017": 13326}, "Total revenue": {"2019": 888190, "2018": 716994, "2017": 519681}}
subscription_revenue_2019 = df["Subscription service revenue"]["2019"]
subscription_revenue_2018 = df["Subscription service revenue"]["2018"]
answer = subscription_revenue_2019 - subscription_revenue_2018