Find the number of ways to partition S = {1, 2, 3, . . . , 2020} into two disjoint sets A and B with A ∪ B = S so that if you choose an element a from A and an element b from B, a + b is never a multiple of 20. A or B can be the empty set, and the order of A and B doesn’t matter. In other words, the pair of sets (A, B) is indistinguishable from the pair of sets (B, A).