What is the percentage change in the number of shares (or units) purchased by the company between August and September 2019?
{"July 1-31, 2019": {"(a) Total Number of Shares (or Units) Purchased": 293.0, "(b) Average Price Paid per Share (or Unit)": 99.35, "(c) Total Number of Shares (or Units) Purchased as Part of Publicly Announced Plans or Programs": 293.0, "(d) Maximum Number (or Approximate Dollar Value) of Shares (or Unites) that May Yet Be Purchased Under the Plans or Programs*": 74706.0}, "August 1 - 31, 2019": {"(a) Total Number of Shares (or Units) Purchased": 39769.0, "(b) Average Price Paid per Share (or Unit)": 107.71, "(c) Total Number of Shares (or Units) Purchased as Part of Publicly Announced Plans or Programs": 39769.0, "(d) Maximum Number (or Approximate Dollar Value) of Shares (or Unites) that May Yet Be Purchased Under the Plans or Programs*": 34937.0}, "September 1 - 30, 2019": {"(a) Total Number of Shares (or Units) Purchased": 91.0, "(b) Average Price Paid per Share (or Unit)": 75.51, "(c) Total Number of Shares (or Units) Purchased as Part of Publicly Announced Plans or Programs": 91.0, "(d) Maximum Number (or Approximate Dollar Value) of Shares (or Unites) that May Yet Be Purchased Under the Plans or Programs*": 34846.0}, "Total": {"(a) Total Number of Shares (or Units) Purchased": 40153.0, "(b) Average Price Paid per Share (or Unit)": 107.58, "(c) Total Number of Shares (or Units) Purchased as Part of Publicly Announced Plans or Programs": 40153.0, "(d) Maximum Number (or Approximate Dollar Value) of Shares (or Unites) that May Yet Be Purchased Under the Plans or Programs*": 34846.0}}
shares_2019 = df["September 1 - 30, 2019"]["(a) Total Number of Shares (or Units) Purchased"]
shares_2018 = df["August 1 - 31, 2019"]["(a) Total Number of Shares (or Units) Purchased"]
answer = (shares_2019 - shares_2018) / shares_2018 * 100.0