What is the difference in total operating lease commitments between 2018 and 2019?
{"": {"2019": "$M", "2018": "$M"}, "Estimated capital expenditure under firm contracts, payable: -- Not later than one year": {"2019": 398, "2018": 416}, "Estimated capital expenditure under firm contracts, payable: -- Later than one year, not later than two years": {"2019": "\u2013", "2018": "\u2013"}, "Estimated capital expenditure under firm contracts, payable: -- Later than two years, not later than five years": {"2019": "\u2013", "2018": "\u2013"}, "Estimated capital expenditure under firm contracts, payable: -- Total capital expenditure commitments": {"2019": 398, "2018": 416}, "Future minimum rentals under non-cancellable operating leases, payable: -- Not later than one year": {"2019": 1998, "2018": 2089}, "Future minimum rentals under non-cancellable operating leases, payable: -- Later than one year, not later than five years": {"2019": 7415, "2018": 7484}, "Future minimum rentals under non-cancellable operating leases, payable: -- Later than five years": {"2019": 12378, "2018": 13331}, "Future minimum rentals under non-cancellable operating leases, payable: -- Total operating lease commitments": {"2019": 21791, "2018": 22904}, "Future minimum rentals under non-cancellable operating leases, payable: -- Total commitments for expenditure": {"2019": 22189, "2018": 23320}}
total_commitments_2018 = df["Future minimum rentals under non-cancellable operating leases, payable: -- Total operating lease commitments"]["2018"]
total_commitments_2019 = df["Future minimum rentals under non-cancellable operating leases, payable: -- Total operating lease commitments"]["2019"]
answer = total_commitments_2018 - total_commitments_2019