Fill in the numbers 1 to 9 in the 3*3 grid so that the numbers on the left are larger than those on the right, and the numbers on the top are larger than those on the bottom. How many different filling methods are there?
在3*3方格表内填人数字1至9，使得左边的数比右边的大，上边的数比下边的大，一共有多少种不同的填法？
<gadget id="calculator">2 * 6</gadget>
<output>12</output>

<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<gadget id="calculator">12 + 12 + 18</gadget>
<output>42</output>

<result>42</result>