What is the difference in the number of shares between class A and class B?
{"Capital stock -- Parent Company": {"Number of shares": "Number of shares", "Capital stock (SEK million)": "Capital stock (SEK million)"}, "Capital stock -- Class A shares": {"Number of shares": 261755983, "Capital stock (SEK million)": 1309}, "Capital stock -- Class B shares": {"Number of shares": 3072395752, "Capital stock (SEK million)": 15363}, "Capital stock -- Total": {"Number of shares": 3334151735, "Capital stock (SEK million)": 16672}}
class_b_shares = df["Capital stock -- Class B shares"]["Number of shares"]
class_a_shares = df["Capital stock -- Class A shares"]["Number of shares"]
answer = class_b_shares - class_a_shares