What is the change in materials and purchased parts from 2018 to 2019?
{"Finished products": {"2019": 10905, "2018": 7099}, "Work in process and inventoried costs under long-term contracts": {"2019": 46951, "2018": 63169}, "Materials and purchased parts": {"2019": 48938, "2018": 23710}, "Customer advances": {"2019": "\u2014", "2018": -9779}, "Net inventories": {"2019": 106794, "2018": 84199}}
materials_and_purchased_parts_2019 = df["Materials and purchased parts"]["2019"]
materials_and_purchased_parts_2018 = df["Materials and purchased parts"]["2018"]
answer = materials_and_purchased_parts_2019 - materials_and_purchased_parts_2018