Ancient Greece believed that: if a number is exactly equal to the sum of all its divisors (except itself), then this number is a "perfect number". For example: 6 has four divisors 1, 2, 3, and 6, and besides 6 itself, there are three divisors of 1, 2, and 3. 6=1+2+3 is exactly the sum of all divisors, 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约数(本身除外)相加之和，那么这个数就是"完全数"．例如：6有四个约数1、2、3、6，除本身6以外，还有1、2、3三个约数．6=1+2+3，恰好是所有约数之和，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>