What is the total area, in square feet, of the Company’s real property?
{"Aurora, IL": {"Purpose": "Corporate headquarters, office, distribution and manufacturing", "Square footage ": 83000, "Termination calendar year": 2020, "Segment": ""}, "Dublin, OH": {"Purpose": "Design center", "Square footage ": 9465, "Termination calendar year": 2019, "Segment": "ISM"}, "Manchester, NH": {"Purpose": "IBW office", "Square footage ": 2287, "Termination calendar year": 2020, "Segment": "IBW"}}
total_area = df["Aurora, IL"]["Square footage "] + df["Dublin, OH"]["Square footage "] + df["Manchester, NH"]["Square footage "]