What is the change in operating margin between 2018 and 2019?
{"Operating margin": {"2018": 4.4, "2019": 5.0, "Ambition": 12.0}, "financial net, tax and other": {"2018": -3.8, "2019": -2.5, "Ambition": -4.0}, "+ depreciation and amortization": {"2018": 3.9, "2019": 2.9, "Ambition": 3.0}, "+ depreciation of leased assets": {"2018": "\u2013", "2019": 1.1, "Ambition": 1}, " change in working capital": {"2018": 3.7, "2019": 1.0, "Ambition": 0.0}, "- capex": {"2018": 2.4, "2019": -2.8, "Ambition": -2.0}, "- leasing payment": {"2018": "-", "2019": -1.3, "Ambition": -1}, "- restructuring costs1)": {"2018": -3.8, "2019": -0.4, "Ambition": -1.0}, "Free cash flow (before M&A)": {"2018": 2.0, "2019": 3.4, "Ambition": 8.0}, "- M&A": {"2018": -0.6, "2019": -0.7, "Ambition": 1.0}}
operating_margin_2019 = df["Operating margin"]["2019"]
operating_margin_2018 = df["Operating margin"]["2018"]
answer = operating_margin_2019 - operating_margin_2018