What was the change in the total number of permanent employees from 2018 to 2019?
{"Total staff costs -- Staff costs included in operating expenses": {"2019": 8.1, "2018": 9.3, "2017": 9.2}, "Total staff costs -- Staff costs included in administrative expenses": {"2019": 37.7, "2018": 36.9, "2017": 34.6}, "Total staff costs -- Total": {"2019": 45.8, "2018": 46.2, "2017": 43.8}, "Staff costs comprise the following -- Wages and salaries": {"2019": 37.2, "2018": 38.1, "2017": 36.4}, "Staff costs comprise the following -- Share-based compensation": {"2019": 1.9, "2018": 2.1, "2017": 1.9}, "Staff costs comprise the following -- Pension costs": {"2019": 3.5, "2018": 3.3, "2017": 3.1}, "Staff costs comprise the following -- Other social security costs": {"2019": 0.9, "2018": 0.6, "2017": 0.3}, "Staff costs comprise the following -- Other staff costs": {"2019": 2.3, "2018": 2.1, "2017": 2.1}, "Staff costs comprise the following -- Total": {"2019": 45.8, "2018": 46.2, "2017": 43.8}, "Average number of permanent employees -- Seafarers": {"2019": 107.6, "2018": 111.7, "2017": 130.6}, "Average number of permanent employees -- Land-based": {"2019": 313.5, "2018": 302.2, "2017": 286.6}, "Average number of permanent employees -- Total": {"2019": 421.1, "2018": 413.9, "2017": 417.2}}
total_2019 = df["Average number of permanent employees -- Total"]["2019"]
total_2018 = df["Average number of permanent employees -- Total"]["2018"]
answer = total_2019 - total_2018