What was the change in Work in progress in 2019 from 2018?
{"": {"2019": "", "2018": "(in millions)"}, "Raw materials": {"2019": 260, "2018": 276}, "Work in progress": {"2019": 739, "2018": 656}, "Finished goods": {"2019": 837, "2018": 925}, "Inventories": {"2019": 1836, "2018": 1857}}
work_in_progress_2019 = df["Work in progress"]["2019"]
work_in_progress_2018 = df["Work in progress"]["2018"]
answer = work_in_progress_2019 - work_in_progress_2018