What is the change in Sales Mix of Software between 2018 and 2019?
{"Hardware": {"2019": 622949, "2018": 653499, "%Change": -5}, "Software": {"2019": 753729, "2018": 736509, "%Change": 2}, "Services": {"2019": 149966, "2018": 140233, "%Change": 7}, "": {"2019": 1526644, "2018": 1530241, "%Change": "\u2014"}}
software_2019 = df["Software"]["2019"]
software_2018 = df["Software"]["2018"]
answer = software_2019 - software_2018