What was the change in the Number of common shares repurchased in 2019 from 2018?
{"": {"2019": "", "2018": "(in millions)", "2017": ""}, "Number of common shares repurchased": {"2019": 12, "2018": 10, "2017": 8}, "Repurchase value": {"2019": 1014, "2018": 966, "2017": 621}}
number_of_common_shares_repurchased_2019 = df["Number of common shares repurchased"]["2019"]
number_of_common_shares_repurchased_2018 = df["Number of common shares repurchased"]["2018"]
answer = number_of_common_shares_repurchased_2019 - number_of_common_shares_repurchased_2018