Sanchahe Town holds a basketball games for elementary school teachers. Each school forms a team, and every two teams play one game. How many games will the 13 schools in Group B play in total?
三岔河镇举行小学教师篮球运动会，每校组建一支队伍，每两队打一场，B组的13家学校共打多少场比赛．
<gadget id="calculator">13 - 1</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">13 * 6</gadget>
<output>78</output>

<result>78</result>