What was the percentage change in operating income in 2018 from 2017?
{"Net sales": {"2018": 2157.7, "2017": 1797.6}, "Less: surcharge revenue": {"2018": 365.4, "2017": 239.2}, "Net sales excluding surcharge revenue": {"2018": 1792.3, "2017": 1558.4}, "Operating income": {"2018": 189.3, "2017": 121.5}, "Special items: -- Loss on divestiture of business": {"2018": "\u2014", "2017": 3.2}, "Special items: -- Adjusted operating income excluding special items": {"2018": 189.3, "2017": 124.7}, "Special items: -- Operating margin": {"2018": 8.8, "2017": 6.8}, "Special items: -- Adjusted operating margin excluding surcharge revenue and special items": {"2018": 10.6, "2017": 8.0}}
operating_income_2018 = df["Operating income"]["2018"]
operating_income_2017 = df["Operating income"]["2017"]
answer = ( operating_income_2018 - operating_income_2017 ) / operating_income_2017 * 100.0