What was the increase / (decrease) in the volume-related revenue from 2018 to 2019?
{"Volume-related": {"2019": 27.5, "2018": 16.5}, "Maintenance services": {"2019": 0.1, "2018": 0.1}, "Total revenue from contracts with customers": {"2019": 27.6, "2018": 16.6}, "RNG incentives": {"2019": 0.5, "2018": 1.3}, "Alternative fuel tax credit": {"2019": 10.6, "2018": 2.6}, "Other revenue": {"2019": 0.3, "2018": 0.2}, "Total Energy segment revenue": {"2019": 39.0, "2018": 20.7}}
volumerelated_2019 = df["Volume-related"]["2019"]
volumerelated_2018 = df["Volume-related"]["2018"]
answer = volumerelated_2019 - volumerelated_2018