What was the percentage change in social security costs in 2019 from 2018?
{"": {"2019": "\u00a3m", "2018": "\u00a3m"}, "Wages and salaries": {"2019": 345.6, "2018": 325.9}, "Social security costs": {"2019": 71.6, "2018": 58.7}, "Pension costs": {"2019": 21.5, "2018": 19.3}, "Total payroll costs": {"2019": 438.7, "2018": 403.9}}
social_security_2019 = df["Social security costs"]["2019"]
social_security_2018 = df["Social security costs"]["2018"]
answer = ( social_security_2019 - social_security_2018 ) / social_security_2018 * 100.0