Combinatorics has always been my weakest point, I want to improve it.
There are such problems like:
"Five friends should give each other gifts. They have made a gift each, as they should give away to one of the others. In how many ways can they distribute gifts such that everyone gets a gift, and no one gets the gift they have made?"
From a specfic country's olympiad.
Can somebody tell me, which text, or what resource to use to be able to learn these types of questions?