Sandwiches
It is easy to find some solutions. Can you be sure you have found
them all?
Start with 1, 1, 2, 2, 3 and 3. Then experiment until you find a
solution.
Now try 1, 1, 2 and 2. Is it possible? If not, why not?
What about 1, 1, 2, 2, 3, 3, 4 and 4?