Input (grade 13): In mathematics, a perfect number is defined as a positive integer which is the sum of its proper positive divisors, that is, the sum of the positive divisors excluding the number itself.
Output (grade 12): A number is called a perfect number if by adding all the positive divisors of the number (except itself), the result is the number itself. A positive integer such that every smaller positive integer is a sum of distinct divisors of it is a practical number.