What is the change in Time and bareboat charter revenues from Years Ended December 31, 2018 to 2019?
{"Time and bareboat charter revenues": {"2019": 263683, "2018": 213923}, "Voyage charter revenues(1)": {"2019": 33275, "2018": 83542}, "Contracts of affreightment revenues": {"2019": 58589, "2018": 68698}, "Total shipping revenues": {"2019": 355547, "2018": 366163}}
time_and_bareboat_charter_revenues_2019 = df["Time and bareboat charter revenues"]["2019"]
time_and_bareboat_charter_revenues_2018 = df["Time and bareboat charter revenues"]["2018"]
answer = time_and_bareboat_charter_revenues_2019 - time_and_bareboat_charter_revenues_2018