During the APEC meeting, the leaders of 9 countries shook hands with each other. Every two people shook hands once. How many times did they shake hands in total?
APEC会议期间，有9个国家领导人互相握手，每两个人要握一次手，一共握了多少次．
<gadget id="calculator">9 - 1</gadget>
<output>8</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">8 * (9/2)</gadget>
<output>36</output>

<result>36</result>