What was the change in the total net gross freight receivables in 2019 from 2018?
{"Gross freight receivables: -- Not yet due": {"2019": 39.8, "2018": 44.0, "2017": 25.5}, "Gross freight receivables: -- Due < 30 days": {"2019": 22.5, "2018": 18.8, "2017": 26.0}, "Gross freight receivables: -- Due between 30 and 180 days": {"2019": 25.3, "2018": 20.5, "2017": 18.4}, "Gross freight receivables: -- Due > 180 days": {"2019": 6.0, "2018": 4.4, "2017": 2.7}, "Gross freight receivables: -- Total gross": {"2019": 93.6, "2018": 87.7, "2017": 72.6}, "Gross freight receivables: -- Allowance for expected credit loss": {"2019": 3.7, "2018": 1.7, "2017": 1.3}, "Gross freight receivables: -- Total net": {"2019": 89.9, "2018": 86.0, "2017": 71.3}}
gross_freight_receivables_total_net_2019 = df["Gross freight receivables: -- Total net"]["2019"]
gross_freight_receivables_total_net_2018 = df["Gross freight receivables: -- Total net"]["2018"]
answer = gross_freight_receivables_total_net_2019 - gross_freight_receivables_total_net_2018