Among the four-digit number, how many are equal to the original number and its inverse number (for example, the inverse number of 123 is 321, the inverse number of 306 is 603, ...).
四位数中，原数与其反序数(例如，123的反序数是321，306的反序数是603，…)相等的有多少个．
<gadget id="calculator">9 * 10</gadget>
<output>90</output>

<result>90</result>