There are 10 athletes on the sports field. Every two athletes shake hands once. How many handshakes are there in total?
运动场上10名运动员，每两名运动员之间都要握一次手，一共要握手多少次．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">10 * (9/2)</gadget>
<output>45</output>

<result>45</result>