What was the increase ./ (decrease) in the cost of sales from 2018 to 2019?
{"Cost of sales": {"December 31, 2019": 22, "December 31, 2018": 23, "December 31, 2017": 12}, "Selling, general and administrative": {"December 31, 2019": 46, "December 31, 2018": 67, "December 31, 2017": 31}, "Research and development": {"December 31, 2019": 77, "December 31, 2018": 35, "December 31, 2017": 18}, "Total pre-payroll tax and social contribution compensation": {"December 31, 2019": 145, "December 31, 2018": 125, "December 31, 2017": 61}}
cost_of_sales_2019 = df["Cost of sales"]["December 31, 2019"]
cost_of_sales_2018 = df["Cost of sales"]["December 31, 2018"]
answer = cost_of_sales_2019 - cost_of_sales_2018