What was the change in the Deferred costs from 2018 to 2019?
{"Prepaid expenses": {"July 31, 2019 ": 2640, "July 31, 2018": 2476}, "Contract costs": {"July 31, 2019 ": 23375, "July 31, 2018": "\u2014"}, "Deferred costs": {"July 31, 2019 ": 8867, "July 31, 2018": 9377}, "Strategic investments": {"July 31, 2019 ": 10672, "July 31, 2018": 10672}, "Other assets": {"July 31, 2019 ": 45554, "July 31, 2018": 22525}}
deferred_cost_2019 = df["Deferred costs"]["July 31, 2019 "]
deferred_cost_2018 = df["Deferred costs"]["July 31, 2018"]
answer = deferred_cost_2019 - deferred_cost_2018