What was the change in the calendar days less scheduled off-hire days?
{"Total operating revenues": {"2019": 230654, "2018": 302979, "Change": -72325, "% Change": -24}, "Vessel operating expenses": {"2019": -67601, "2018": -70543, "Change": 2942, "% Change": -4}, "Voyage, charterhire and commission expenses (including expenses from collaborative arrangement)": {"2019": -38381, "2018": -104463, "Change": 66082, "% Change": -63}, "Administrative expenses": {"2019": -50801, "2018": -51716, "Change": 915, "% Change": -2}, "Project development expenses": {"2019": -2050, "2018": -5165, "Change": 3115, "% Change": -60}, "Depreciation and amortization": {"2019": -64945, "2018": -65496, "Change": 551, "% Change": -1}, "Impairment of long-term assets": {"2019": -42098, "2018": "\u2014", "Change": -42098, "% Change": 100}, "Other operating gains": {"2019": 13295, "2018": 50740, "Change": -37445, "% Change": -74}, "Operating (loss)/income": {"2019": -21927, "2018": 56336, "Change": -78263, "% Change": -139}, "Equity in net losses of affiliates": {"2019": -22565, "2018": -138677, "Change": 116112, "% Change": -84}, "Other Financial Data: -- Average Daily TCE (1) (to the closest $100)": {"2019": 44400, "2018": 43700, "Change": 700, "% Change": 2}, "Other Financial Data: -- Calendar days less scheduled off-hire days": {"2019": 3840, "2018": 3987, "Change": -147, "% Change": -4}}
cal_days_2018 = df["Other Financial Data: -- Calendar days less scheduled off-hire days"]["2018"]
cal_days_2019 = df["Other Financial Data: -- Calendar days less scheduled off-hire days"]["2019"]
answer = cal_days_2018 - cal_days_2019