For a non-zero natural number n, if you can find a non-zero natural number a, b such that n=a+b+ab, then n is called a "association number", such as: 3=1+1+1*1, then 3 is a "Friendship number", then among the 20 natural numbers from 1 to 20, how many "friendship numbers" are there?
对于非零自然数n，如果能找到非零自然数a，b使得n=a+b+ab，则称n是一个"联谊数"，如：3=1+1+1*1，则3就是一个"联谊数"，那么从1到20这20个自然数当中，"联谊数"共有多少个．
<gadget id="calculator">1 * 1</gadget>
<output>1</output>

<gadget id="calculator">9 + 1 + 1 + 1</gadget>
<output>12</output>

<result>12</result>