What is the percentage increase in the stock based compensation expense on cost of sales?
{"": {"September 30, 2019": "(Amounts in thousands)", "September 30, 2018": ""}, "Cost of sales": {"September 30, 2019": 7, "September 30, 2018": 5}, "Engineering and development": {"September 30, 2019": 49, "September 30, 2018": 32}, "Selling, general and administrative": {"September 30, 2019": 736, "September 30, 2018": 654}, "Total": {"September 30, 2019": 792, "September 30, 2018": 691}}
cost_of_sales_2019 = df["Cost of sales"]["September 30, 2019"]
cost_of_sales_2018 = df["Cost of sales"]["September 30, 2018"]
answer = ( cost_of_sales_2019 - cost_of_sales_2018 ) / cost_of_sales_2018 * 100.0