What is the change in number of basic shares between 2017 and 2018?
{"Net income   ": {"2019": 85762.0, "2018": 104690.0, "2017": 116481.0}, "Earnings per share -- Basic   ": {"2019": 1.33, "2018": 1.58, "2017": 1.68}, "Earnings per share -- Effect of CESs     ": {"2019": -0.01, "2018": "-", "2017": "-"}, "Earnings per share -- Diluted   ": {"2019": 1.32, "2018": 1.58, "2017": 1.68}, "Weighted average number of shares: -- Basic     ": {"2019": 64397.0, "2018": 66201.0, "2017": 69175.0}, "Weighted average number of shares: -- Effect of CESs     ": {"2019": 706.0, "2018": 233.0, "2017": 249.0}, "Weighted average number of shares: -- Diluted     ": {"2019": 65103.0, "2018": 66434.0, "2017": 69424.0}}
basic_shares_2017 = df["Weighted average number of shares: -- Basic     "]["2017"]
basic_shares_2018 = df["Weighted average number of shares: -- Basic     "]["2018"]
answer = basic_shares_2017 - basic_shares_2018