Two numbers are amicable if the sum of the proper divisors of one equals the other number, and vice versa. For example, (220, 284) is an amicable pair, since the proper divisors of 220 add up to 284 (1 + 2 + 4 + 5 + 10 + 11 + 20 + 22 + 44 + 55 + 110 = 284), and the proper divisors of 284 add up to 220 (1 + 2 + 4 + 71 + 142 = 220).