What was the percentage change in the amount of surcharge revenue from 2018 to 2019?
{"Net sales": {"2019": 2380.2, "2018": 2157.7}, "Less: surcharge revenue": {"2019": 438.1, "2018": 365.4}, "Net sales excluding surcharge revenue": {"2019": 1942.1, "2018": 1792.3}, "Operating income": {"2019": 241.4, "2018": 189.3}, "Special items: -- Acquisition-related costs": {"2019": 1.2, "2018": "\u2014"}, "Special items: -- Adjusted operating income excluding special items": {"2019": 242.6, "2018": 189.3}, "Special items: -- Operating margin": {"2019": 10.1, "2018": 8.8}, "Special items: -- Adjusted operating margin excluding surcharge revenue and special items": {"2019": 12.5, "2018": 10.6}}
less_surcharge_revenue_2019 = df["Less: surcharge revenue"]["2019"]
less_surcharge_revenue_2018 = df["Less: surcharge revenue"]["2018"]
answer = ( less_surcharge_revenue_2019 - less_surcharge_revenue_2018 ) / less_surcharge_revenue_2018 * 100.0