I know it's $$$\Theta\left(n^2\right)$$$ but I don't know how to build a case to hack it.

I read Blowing up unordered_map, and how to stop getting hacked on it but it doesn't work for GNU C++20 (64). Custom invocation of the hack shown in the blog shows:

x = 107897: 0.064 seconds, sum = 2666686666700000
x = 126271: 0.055 seconds, sum = 2666686666700000

which means that this hack doesn't work.