The number 916238457 is an example of a 9-digit number that contains each of the digits 1 through 9 exactly once. It also has the property that the numbers 1 to 5 appear in the normal order, but 1 to 6 do not appear in the normal order. Ask how many such numbers there are.
数916238457是一个包含1至9每个数字恰好各一次的9位数的例子．它还具有性质：数字1至5以正常的顺序出现在其中，但1至6不以正常的顺序出现．问这样的数有多少个．
<gadget id="calculator">5 * 7 * 8 * 9</gadget>
<output>2_520</output>

<result>2_520</result>