What is the change in the proportion of gross debt with interest rate protection from 2018 to 2019?
{"Debt to assets ratio": {"Notes": "A", "2019": 67.8, "2018": 53.1, "Change": 14.7}, "Interest cover": {"Notes": "B", "2019": 1.67, "2018": 1.91, "Change": 0.24}, "Weighted average debt maturity": {"Notes": "", "2019": 5.0, "2018": 5.8, "Change": 0.8}, "Weighted average cost of gross debt (excluding RCF)": {"Notes": "", "2019": 4.3, "2018": 4.2, "Change": 0.1}, "Proportion of gross debt with interest rate protection": {"Notes": "", "2019": 88.0, "2018": 84.0, "Change": 4.0}, "Immediately available cash and facilities": {"Notes": "C", "2019": 241.5, "2018": 246.8, "Change": -5.3}}
proportion_2019 = df["Proportion of gross debt with interest rate protection"]["2019"]
proportion_2018 = df["Proportion of gross debt with interest rate protection"]["2018"]
answer = proportion_2019 - proportion_2018