What was the change in purchase commitments that were less than 1 year between 2018 and 2019?
{"Less than 1 year": {"July 27, 2019": 4239, "July 28, 2018": 5407}, "1 to 3 years": {"July 27, 2019": 728, "July 28, 2018": 710}, "3 to 5 years": {"July 27, 2019": "\u2014", "July 28, 2018": 360}, "Total": {"July 27, 2019": 4967, "July 28, 2018": 6477}}
less_than_1_year_2019 = df["Less than 1 year"]["July 27, 2019"]
less_than_1_year_2018 = df["Less than 1 year"]["July 28, 2018"]
answer = less_than_1_year_2019 - less_than_1_year_2018