What was the change in the Basic weighted average number of shares (millions) from 2018 to 2019?
{"Basic weighted average number of shares (millions)": {"2019": 9912.0, "2018": 9911.0, "2017": 9938.0}, "Dilutive shares from share options (millions)": {"2019": 6.0, "2018": 2.0, "2017": 27.0}, "Dilutive shares from executive share awards (millions)": {"2019": 57.0, "2018": 48.0, "2017": 29.0}, "Diluted weighted average number of shares (millions)": {"2019": 9975.0, "2018": 9961.0, "2017": 9994.0}, "Basic earnings per share": {"2019": 21.8, "2018": 20.5, "2017": 19.2}, "Diluted earnings per share": {"2019": 21.6, "2018": 20.4, "2017": 19.1}}
basic_weighted_average_number_of_shares_millions_2019 = df["Basic weighted average number of shares (millions)"]["2019"]
basic_weighted_average_number_of_shares_millions_2018 = df["Basic weighted average number of shares (millions)"]["2018"]
answer = basic_weighted_average_number_of_shares_millions_2019 - basic_weighted_average_number_of_shares_millions_2018