What was the percentage change in the intensity ratio in 2019 from 2018?
{"Scope 2": {"Combustion of natural gas and operation of owned vehicles": "Electricity consumption in offices", "220.9": 4487.2, "320.0": 4457.3, "251.8": 4681.9}, "Scope 3": {"Combustion of natural gas and operation of owned vehicles": "Business travel (air and car)", "220.9": 3260.9, "320.0": 5117.4, "251.8": 4510.9}, "Intensity ratio -- tCO2e per $M of billings": {"Combustion of natural gas and operation of owned vehicles": "", "220.9": 10.5, "320.0": 12.9, "251.8": 14.9}}
intensity_ratio_2019 = df["Intensity ratio -- tCO2e per $M of billings"]["220.9"]
intensity_ratio_2018 = df["Intensity ratio -- tCO2e per $M of billings"]["320.0"]
answer = ( intensity_ratio_2019 - intensity_ratio_2018 ) / intensity_ratio_2018 * 100.0