What was the change in Extended warranty and service contracts recognized over time between 2017 and 2018?
{"Product and service revenue recognized at point in time": {"2019": 786918, "2018": 715055, "2017": 667440}, "Extended warranty and service contracts recognized over time": {"2019": 2030, "2018": 3837, "2017": 3572}, "Total": {"2019": 788948, "2018": 718892, "2017": 671012}}
extended_warranty_2018 = df["Extended warranty and service contracts recognized over time"]["2018"]
extended_warranty_2017 = df["Extended warranty and service contracts recognized over time"]["2017"]
answer = extended_warranty_2018 - extended_warranty_2017