Naughty invited 5 classmates to his birthday party. If they shook hands once, how many times did they shake hands in total? Please express it in the way you like.
淘气请来5位同学参加他的生日聚会，如果他们每两人之间握一次手，请问一共握了几次手？请用你喜欢的方法表示出来．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">4 * (5/2)</gadget>
<output>10</output>

<result>10</result>