Perhaps reading this might help ~~ Also not a kata suggestion

I don't understand, how can I solve this?

NICE! bonus points if you wrote them all on notebook paper first

That is clearly stated:

for similar case ( 2, 2, 4 ) - when is posible multiple variants, chose first posible operation from list.

Where list is:

1)addition,
2)subtraction,
3)multiplication,
4)division.

The description does not say what to do in a situation like ( "2 2 4"). 2 and 2 could be addition or multiplication. Forcing my answer to choose addition over multiplication passes the tests, but you should clarify what to do. The same problem exists for subtraction and division.

I did not understand that "123
"
means "123\n". It looks like a space but it is \n. If you do not understand this you cannot complete the challenge. \n counts as a character meaning new line.