What is the change in Leasehold improvements from December 31, 2019 to December 31, 2018?
{"Furniture and equipment": {"Useful life in years": 5, "2019": 1785, "2018": 1189}, "Leasehold improvements (1)": {"Useful life in years": 5, "2019": 4074, "2018": 2776}, "System hardware": {"Useful life in years": 5, "2019": 1596, "2018": 1404}, "Office computers": {"Useful life in years": 3, "2019": 5309, "2018": 3745}, "Computer and system software": {"Useful life in years": 3, "2019": 1451, "2018": 1385}, "": {"Useful life in years": "", "2019": 14215, "2018": 10499}, "Less accumulated depreciation and amortization": {"Useful life in years": "", "2019": -7931, "2018": -5849}, "Property and equipment, net": {"Useful life in years": "", "2019": 6284, "2018": 4650}}
leasehold_improvements_1_2019 = df["Leasehold improvements (1)"]["2019"]
leasehold_improvements_1_2018 = df["Leasehold improvements (1)"]["2018"]
answer = leasehold_improvements_1_2019 - leasehold_improvements_1_2018