Financial report context:
14 . capital stock shares outstanding . the following table presents information regarding capital stock: .

Financial table:
( in thousands ): class a common stock authorized; december 31 , 2017: 1000000; december 31 , 2016: 1000000
( in thousands ): class a common stock issued and outstanding; december 31 , 2017: 339235; december 31 , 2016: 338240
( in thousands ): class b-1 common stock authorized issued and outstanding; december 31 , 2017: 0.6; december 31 , 2016: 0.6
( in thousands ): class b-2 common stock authorized issued and outstanding; december 31 , 2017: 0.8; december 31 , 2016: 0.8
( in thousands ): class b-3 common stock authorized issued and outstanding; december 31 , 2017: 1.3; december 31 , 2016: 1.3
( in thousands ): class b-4 common stock authorized issued and outstanding; december 31 , 2017: 0.4; december 31 , 2016: 0.4

Additional report context:
cme group has no shares of preferred stock issued and outstanding . associated trading rights . members of cme , cbot , nymex and comex own or lease trading rights which entitle them to access open outcry trading , discounts on trading fees and the right to vote on certain exchange matters as provided for by the rules of the particular exchange and cme group 2019s or the subsidiaries 2019 organizational documents . each class of cme group class b common stock is associated with a membership in a specific division for trading at cme . a cme trading right is a separate asset that is not part of or evidenced by the associated share of class b common stock of cme group . the class b common stock of cme group is intended only to ensure that the class b shareholders of cme group retain rights with respect to representation on the board of directors and approval rights with respect to the core rights described below . trading rights at cbot are evidenced by class b memberships in cbot , at nymex by class a memberships in nymex and at comex by comex division memberships . members of cbot , nymex and comex do not have any rights to elect members of the board of directors and are not entitled to receive dividends or other distributions on their memberships or trading permits . core rights . holders of cme group class b common shares have the right to approve changes in specified rights relating to the trading privileges at cme associated with those shares . these core rights relate primarily to trading right protections , certain trading fee protections and certain membership benefit protections . votes on changes to these core rights are weighted by class . each class of class b common stock has the following number of votes on matters relating to core rights : class b-1 , six votes per share ; class b-2 , two votes per share ; class b-3 , one vote per share ; and class b-4 , 1/6th of one vote per share . the approval of a majority of the votes cast by the holders of shares of class b common stock is required in order to approve any changes to core rights . holders of shares of class a common stock do not have the right to vote on changes to core rights . voting rights . with the exception of the matters reserved to holders of cme group class b common stock , holders of cme group common stock vote together on all matters for which a vote of common shareholders is required . in these votes , each holder of shares of class a or class b common stock of cme group has one vote per share . transfer restrictions . each class of cme group class b common stock is subject to transfer restrictions contained in the certificate of incorporation of cme group . these transfer restrictions prohibit the sale or transfer of any shares of class b common stock separate from the sale of the associated trading rights . election of directors . the cme group board of directors is currently comprised of 20 members . holders of class b-1 , class b-2 and class b-3 common stock have the right to elect six directors , of which three are elected by class b-1 shareholders , two are elected by class b-2 shareholders and one is elected by class b-3 shareholders . the remaining directors are elected by the class a and class b shareholders voting as a single class. .

Question: how many directors can be elected by the class b-1 and class b-2 shareholders?
Answer: 5