Yop, waiting for C++20 full compatible compiler in Codewars (C++17 supported), rangev3 is supported as external library .. See github repository "using rangeV3 in codewars".
A bit verbose in the header files, but more convenient...
Was tired to type "c.begin(),c.end()"; and the ::views can be really nice on some algos.

Ranges library has first been introduced in c++20.

minor correction: use const reference, to make sure you are not changing anything on the data structure.

Whenever passing vectors, or any STL data structure to a function, always use references.

Use Pass by reference instead of Pass by Value.

This would reduce the time in copying your vector.

This is a bad calculator since there is no lexical analysis (input would be "10 + 10"). Try making it better and expand on it so it supports powers (**), lets you use more that one expression ("10 + 10 / 2) WITH the order of calculation. Sure you can do some good things with it.

-4 / 2 = -2

why checking a>=2?

-4 / 2 = 2.

I did not think that someone will find this solution so quickly haha

Your kata won't leave beta. Please consider these guidelines for your next kata:
https://docs.codewars.com/authoring/guidelines/kata/

spec got updated, but still :/

you should specify such information in the description

such an array is not possible in this kata because all arrays in this kata provide for the presence of digits from 1 to some N, but not in sorted order

Can there even be duplicates in the input?

You have placed your random tests in preloaded section. That's not the place for random tests.

pairs like 2+2 or 3+3 just as true

ok, so how many pairs are there in this array then?

[4, 2, 2] -> 1 or 3 ?