What was the change in the gross Total property, plant and equipment held under capital leases between 2018 and 2019?
{"Buildings and improvements": {"2019": 23717, "2018": 23717}, "Machinery and equipment ": {"2019": 12293, "2018": 10995}, "Capital assets in progress": {"2019": 11831, "2018": 7747}, "Total property, plant and equipment held under capital leases, gross": {"2019": 47841, "2018": 42459}, "Less: accumulated amortization ": {"2019": -8762, "2018": -6123}, "Total property, plant and equipment held under capital leases, net": {"2019": 39079, "2018": 36336}}
total_property_plant_and_equipment_held_under_capital_leases_gross_2019 = df["Total property, plant and equipment held under capital leases, gross"]["2019"]
total_property_plant_and_equipment_held_under_capital_leases_gross_2018 = df["Total property, plant and equipment held under capital leases, gross"]["2018"]
answer = total_property_plant_and_equipment_held_under_capital_leases_gross_2019 - total_property_plant_and_equipment_held_under_capital_leases_gross_2018