I think I have solved this one, but I've got 2 different solutions.
First solution (I think this is the right one)
I just noticed a pattern where the numbers between the 1s just alternate like this: we have the first one made by 1 then 2, 3 and -1 ---> 1, 2, 3, -1.
Then we should alternate 2, 3 and it becomes -1, 3, 2, 1.
And then we have the $x, x$. Considering that it always alternates we can just write 2,3 instead of x.So we get something like this:
I just think the number ones are what you should add to the numbers.
We have 1 at the beginning. 1+1=2 and 2+1=3
Then we have to rewrite 3, and do 3-1=2, then 2-1=1.
Then we should replace the x with 2 and then 2+1=3 so it would be something like this:
Can you confirm one of these theories or come up with a better solution?