What was the change in properties from start to end 2019?
{"As of January 1, 2019": {"Vessel": 206753, "Vessel Equipment": 2630, "Properties": 4969, "Other": 19, "Total": 214371}, "Additions": {"Vessel": 1001, "Vessel Equipment": 336, "Properties": 1080, "Other": 47, "Total": 2464}, "Depreciation expense": {"Vessel": -7722, "Vessel Equipment": -1109, "Properties": -1499, "Other": -10, "Total": -10340}, "As of December 31, 2019": {"Vessel": 200032, "Vessel Equipment": 1857, "Properties": 4550, "Other": 56, "Total": 206495}}
properties_2019 = df["As of January 1, 2019"]["Properties"]
properties_2018 = df["As of December 31, 2019"]["Properties"]
answer = properties_2019 - properties_2018