There are 5 teachers shaking hands with each other in the meeting place, every two shake hands once, how many times they shake hands in total.
有5名老师在会场上相互握手，每两人握手一次，她们一共握手多少次．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<result>10</result>