What is the change in Cost of revenue from Year Ending March 31, 2018 to 2019?
{"Cost of revenue ": {"2019": 1684, "2018": 1053, "2017": 1353}, "Research and development  ": {"2019": 6199, "2018": 2555, "2017": 1873}, "Sales and marketing  ": {"2019": 7856, "2018": 4477, "2017": 4719}, "General and administrative  ": {"2019": 10215, "2018": 3649, "2017": 2349}, "Total share-based compensation expense": {"2019": 25954, "2018": 11734, "2017": 10294}}
cost_2019 = df["Cost of revenue "]["2019"]
cost_2018 = df["Cost of revenue "]["2018"]
answer = cost_2019 - cost_2018