If there are at least two numbers in a natural number, and each number is smaller than each number to its left, then this number is called "descending number". How many "descending numbers" are there in the 2 to 6 digits composed of 6 numbers 1, 2, 3, 4, 5, 6?
如果一个自然数中至少有两个数字，且每个数字小于其左边的每个数字，则称这个数是"下降数"．由1，2，3，4，5，6这6个数字组成的2～6位数中的"下降数"共有多少个．
<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<gadget id="calculator">10 + 6 + 3 + 1</gadget>
<output>20</output>

<gadget id="calculator">1 + 6 + 15 + 15 + 20</gadget>
<output>57</output>

<result>57</result>