Why "1 2 3 4" is similar to "1 2 3 4 2" ans is not similar to "1 1 2 3 4". The first set is obtained from these two by deleting "2" or "1" respectively. What do I misunderstand?

"1 2 3 4" and "1 1 2 3 4" are similar. Read the task one more time :)

Good luck!

Oh, thanx :)