6 good friends, haven't seen each other for a long time, when we met, everyone shook hands kindly, if every two people shook hands once, how many times did they shake hands in total?
6个好朋友，好久没见了，见面的时候大家都亲切的握手，如果每两人握一次手，他们一共握了几次手呢？
<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>