Похоже, что во входных строчках помимо чисел A и B присутствуют ещё и пробелы. Или другие пустые символы...

You are wrong. Tests contain only two integers with exactly one end of line after each of them (no other characters).

I've got AC after I added .Trim(). But there could also be another unnoticed change in my code which actually fixed it.
So anyway, extra precautions would never do the harm. :)
Thanks for your time examining input validity, Sandro!

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