After my first attempt brute forcing it timed out, I went all in on reducing the number of times I'm applying the formula. I found upper and lower bounds within orders of magnitude, then halfstepped between them until they're only one apart and tested both. It's something like 50 lines of code, and I get in only to find... half ya'll brute forced it too, just better than me.

With a few ridiculously high numbers, it's conceivable you could make brute forcing not viable, and release this as a higher level kata.