There are 9 students who want to play chess. They are going to be divided into two groups, people in different groups will only play one game with each other, and people in the same group will not play. How many games can they play in total?
有9个同学要进行象棋比赛．他们准备分成两组，不同组的人相互之间只比赛一场，同组的人之间不比赛．他们一共最多能比赛多少场？
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<result>20</result>