What was the change in accrued purchases from 2018 to 2019?
{"Unearned revenue": {"2018": 38680, "2019": 48183}, "Accrued off-hire": {"2018": 7376, "2019": 6968}, "Accrued purchases": {"2018": 18578, "2019": 9759}, "Accrued interest": {"2018": 38107, "2019": 36746}, "Other accruals": {"2018": 24709, "2019": 34586}, "Total": {"2018": 127450, "2019": 136242}}
accrued_purchases_2019 = df["Accrued purchases"]["2019"]
accrued_purchases_2018 = df["Accrued purchases"]["2018"]
answer = accrued_purchases_2019 - accrued_purchases_2018