Ancient Greece believed that if a number is exactly equal to the sum of all its factors (except itself), then the number is a "perfect number". For example: 6 has four factors 1, 2, 3, 6, besides 6 itself, there are three factors of 1, 2, 3. 6=1+2+3, exactly the sum of all factors, so 6 is a "perfect number". Which of the following numbers is a "perfect number"
古希腊认为：如果一个数恰好等于它的所有因数(本身除外)相加之和，那么这个数就是"完全数"．例如：6有四个因数1、2、3、6，除本身6以外，还有1、2、3三个因数．6=1+2+3，恰好是所有因数之和，所以6就是"完全数"．下面的数中是"完全数"的是多少
<gadget id="calculator">1 + 2 + 4 + 7 + 14</gadget>
<output>28</output>

<result>28</result>