Yesterday, I bought a new calculator in a antique shop. I wanted to try some basic operations to see if it is working well. Everything felt right, except for the + sign: it seems like it has been replaced with an obscure new operator!

At first, I tried these additions that seemed totally fine:

5 + 5 = 10
6 + 5 = 11
39 + 8 = 47
6 + 3 + 7 + 10 = 26

But then, things started to go slightly off with some additional tries:

6 + 4 = 12
7 + 3 + 10 = 21
8 + 4 + 9 = 22

So I computed more operations, and nothing made sense anymore!

2 + 2 = 10
1 + 2 + 3 + 4 = 1
5 + 5 + 5 + 5 + 5 = 130
3000 + 333 + 30 + 3 = 1020

My last hope to try to understand this operator was to compute a random big number to understand what's going on, but all I got was:

17603415 + 30 = ERROR

So, can you tell me what is this operator that replaced the + sign on my calculator?