What was the change in the Software delivery, support and maintenance value from 2018 to 2019?
{"Cost of revenue: -- Software delivery, support and maintenance": {"2019": 2075, "2018": 2184, "2017": 2879}, "Cost of revenue: -- Client services": {"2019": 4067, "2018": 3997, "2017": 4484}, "Cost of revenue: -- Total cost of revenue": {"2019": 6142, "2018": 6181, "2017": 7363}, "Cost of revenue: -- Selling, general and administrative expenses": {"2019": 27348, "2018": 24213, "2017": 23497}, "Cost of revenue: -- Research and development": {"2019": 9200, "2018": 8937, "2017": 8605}, "Cost of revenue: -- Total stock-based compensation expense": {"2019": 42690, "2018": 39331, "2017": 39465}}
cost_of_revenue_software_delivery_support_maintenance_2019 = df["Cost of revenue: -- Software delivery, support and maintenance"]["2019"]
cost_of_revenue_software_delivery_support_maintenance_2018 = df["Cost of revenue: -- Software delivery, support and maintenance"]["2018"]
answer = cost_of_revenue_software_delivery_support_maintenance_2019 - cost_of_revenue_software_delivery_support_maintenance_2018