10 students shake hands once between two students, how many handshakes in total.
10个学生，每两人之间握一次手，一共握手多少次．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

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

<gadget id="calculator">9 * 5</gadget>
<output>45</output>

<result>45</result>