The sum of the digits of a natural number is 6, and the digits are not the same. How many such natural numbers are there?
一个自然数的各位数字的和是6，且各位数字不相同．这样的自然数有多少个？
<gadget id="calculator">3 * 3 * 2</gadget>
<output>18</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<gadget id="calculator">18 + 6 + 4 + 2 + 4 + 2 + 1 + 1</gadget>
<output>38</output>

<result>38</result>