There are 10 old friends who meet each other, and every two people shake hands once, how many handshakes are needed in total.
有10个老朋友见面，每两人握一次手，一共需要握手多少次．
<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>