What is the change in the company's raw materials on March 31, 2018 and 2019?
{"Raw materials": {"March 30, 2019": 118608, "March 31, 2018": 110389}, "Work in process": {"March 30, 2019": 272469, "March 31, 2018": 221137}, "Finished goods": {"March 30, 2019": 120716, "March 31, 2018": 140766}, "Total inventories": {"March 30, 2019": 511793, "March 31, 2018": 472292}}
raw_material_2019 = df["Raw materials"]["March 30, 2019"]
raw_material_2018 = df["Raw materials"]["March 31, 2018"]
answer = raw_material_2019 - raw_material_2018