What is the total stock-based compensation for the cost of revenue in 2018 and 2019?
{"Cost of revenue": {"2019": 78, "2018": 129, "2017": 121}, "Research and development": {"2019": 2242, "2018": 760, "2017": 614}, "Selling, general and administrative": {"2019": 824, "2018": 1012, "2017": 706}, "Total costs and expenses": {"2019": 3144, "2018": 1901, "2017": 1441}}
cost_of_revenue_2018 = df["Cost of revenue"]["2018"]
cost_of_revenue_2019 = df["Cost of revenue"]["2019"]
total_stock_based_compensation = cost_of_revenue_2018 + cost_of_revenue_2019