"Denote by S the set of integers {18,19,...,22}. Count the number of BIJECTIVE functions φ:S→S such that φ(x)≠x for all x∈S."
Above is the question. I understand the total bijective functions possible are 5! but I don't know how to exclude the last bit of the question "such that φ(x)≠x for all x∈S". Is there something that can point me in the right direction?