What was the change in raw materials between 2018 and 2019?
{"Raw materials": {"2019": 8921, "2018": 6396}, "Finished goods": {"2019": 25247, "2018": 16594}, "Total inventory, net": {"2019": 34168, "2018": 22990}}
raw_material_2019 = df["Raw materials"]["2019"]
raw_material_2018 = df["Raw materials"]["2018"]
answer = raw_material_2019 - raw_material_2018