What is the percentage change in cost of revenue between 2018 and 2019?
{"Statements of Operations: -- Revenue": {"2019": 10310.0, "2018": 12629.0, "2017": 12149.0}, "Statements of Operations: -- Cost of revenue": {"2019": 4405.0, "2018": 6295.0, "2017": 6627.0}, "Statements of Operations: -- Gross profit": {"2019": 5905.0, "2018": 6334.0, "2017": 5522.0}, "Operating expenses: -- Research and development": {"2019": 12350.0, "2018": 9948.0, "2017": 9572.0}, "Operating expenses: -- Selling, general and administrative": {"2019": 8918.0, "2018": 9982.0, "2017": 9900.0}, "Operating expenses: -- Loss from operations": {"2019": -15363.0, "2018": -13596.0, "2017": -13950.0}, "Operating expenses: -- Interest expense": {"2019": -350.0, "2018": -108.0, "2017": -115.0}, "Operating expenses: -- Interest income and other expense, net": {"2019": 189.0, "2018": 77.0, "2017": 21.0}, "Operating expenses: -- Loss before income taxes": {"2019": -15524.0, "2018": -13627.0, "2017": -14044.0}, "Operating expenses: -- (Benefit from) Provision for income taxes": {"2019": -80.0, "2018": 152.0, "2017": 87.0}, "Operating expenses: -- Net loss": {"2019": -15444.0, "2018": -13779.0, "2017": -14131.0}, "Net loss per share: (1) -- Basic and diluted": {"2019": -2.02, "2018": -2.16, "2017": -2.56}, "Weighted average shares: -- Basic and diluted": {"2019": 7663.0, "2018": 6365.0, "2017": 5521.0}}
cost_of_revenue_2019 = df["Statements of Operations: -- Cost of revenue"]["2019"]
cost_of_revenue_2018 = df["Statements of Operations: -- Cost of revenue"]["2018"]
answer = ( cost_of_revenue_2019 - cost_of_revenue_2018 ) / cost_of_revenue_2018 * 100.0