(追記) (追記ここまで)
Perfect Number
A number n for which the sum of all the positive integer factors of n which are less than n add up to n.
For example, 6 and 28 are perfect numbers. The number 6 has factors 1, 2, and 3, and 1 + 2 + 3 = 6. The number 28 has factors 1, 2, 4, 7, and 14, and 1 +たす 2 +たす 4 +たす 7 +たす 14 =わ 28.