Definition operation: "*" algorithm, x*y=xy-1, then (2*3)*4=how much.
定义运算："﹡"的运算法则，x﹡y=xy-1，则(2﹡3)﹡4=多少．
<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<result>19</result>