We give a greedy algorithm for describing an enumeration of the set of all natural numbers such that the sum of the first n terms of the sequence is divisible by n for each natural number n. We show that this leads to a bijection f of the set of all natural numbers onto itself that has some nice properties. We also show that the average function of the first n terms of the sequence satisfies a functional equation which completely describes all the properties of the function f. In particular, f turns out to be an involution on the set of all natural numbers.