discrete math

Let M be a randomly chosen non-empty subset of S = {1,2,3,...,2026}. Which of the following is the probability that the product of all the elements of M is even?

The number of bijections f(.) from the set S = {1, 2, 3, 4} to itself such that f(f(n)) = n, for all n ∈ S, is __________. (Answer in integer)