What was the change in the Cost of subscription and SaaS revenue between 2018 and 2019?
{"Cost of license revenue": {"January 31, 2020": 1, "February 1, 2019": 1, "February 2, 2018": 2}, "Cost of subscription and SaaS revenue": {"January 31, 2020": 13, "February 1, 2019": 7, "February 2, 2018": 5}, "Cost of services revenue": {"January 31, 2020": 83, "February 1, 2019": 58, "February 2, 2018": 53}, "Research and development": {"January 31, 2020": 459, "February 1, 2019": 391, "February 2, 2018": 363}, "Sales and marketing": {"January 31, 2020": 293, "February 1, 2019": 226, "February 2, 2018": 205}, "General and administrative": {"January 31, 2020": 168, "February 1, 2019": 117, "February 2, 2018": 84}, "Stock-based compensation": {"January 31, 2020": 1017, "February 1, 2019": 800, "February 2, 2018": 712}, "Income tax benefit": {"January 31, 2020": -347, "February 1, 2019": -253, "February 2, 2018": -232}, "Total stock-based compensation, net of tax": {"January 31, 2020": 670, "February 1, 2019": 547, "February 2, 2018": 480}}
cost_of_subscription_and_saas_revenue_2019 = df["Cost of subscription and SaaS revenue"]["February 1, 2019"]
cost_of_subscription_and_saas_revenue_2018 = df["Cost of subscription and SaaS revenue"]["February 2, 2018"]
answer = cost_of_subscription_and_saas_revenue_2019 - cost_of_subscription_and_saas_revenue_2018