What is the percentage change in the revenue from public works from 2018 to 2019?
{"Agricultural, Commercial, and Industrial (ACI)": {"2019": 28940, "2018": 33193}, "Public Works": {"2019": 12128, "2018": 17986}, "Residential": {"2019": 18762, "2018": 19786}, "Total": {"2019": 59830, "2018": 70965}}
public_works_2019 = df["Public Works"]["2019"]
public_works_2018 = df["Public Works"]["2018"]
answer = ( public_works_2019 - public_works_2018 ) / public_works_2018 * 100.0