Four people stand in a line in order $$A$$ $$B$$ $$C$$ $$D$$. The people then change positions in the line so that \textbf{no one person is left standing in the same position as they were to start with}, for example, $$C$$ $$A$$ $D$ $$B$$. How many possible ways are there of doing this? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Permutations->Basic Applications of Permutations