What was the change in ship management fees income from 2017 to 2018?
{"Management and administrative services revenue": {"2019": 5904, "2018": 6167, "2017": 5711}, "Ship management fees income": {"2019": 1210, "2018": 1400, "2017": 824}, "Debt guarantee compensation (i)": {"2019": 693, "2018": 861, "2017": 775}, "Other (ii)": {"2019": -2, "2018": -247, "2017": 135}, "Total": {"2019": 7805, "2018": 8181, "2017": 7445}}
ship_management_fees_income_2018 = df["Ship management fees income"]["2018"]
ship_management_fees_income_2017 = df["Ship management fees income"]["2017"]
answer = ship_management_fees_income_2018 - ship_management_fees_income_2017