Why do you want to guess exact number? Look at the problem from the different angle, for example you have a combination ab as in examples, and result is 81. If you'll have to guess ANY number, it could be almost anything with two digits. In task description you have condition, that you're only interested in square numbers, and moreover, this number must be the largest possible, this is crucial. Performance is the main problem here, but you'll get over that.

I forget to delete loop :x

Every test takes about 0.00012ms and is passed, but whole execution is timed out (12000ms), somebody had this issue? (c#)