The Smallest String

Abdelmohaymn gives you a string $S$ of length $N$ that contains only **uppercase** and **lowercase** English letters. He also gives you two integers $x$ and $y$, meaning that the letters `'a'` to `'z'` have values $[x, x+1, x+2, \ldots, x+25]$ (in order), and the letters `'A'` to `'Z'` have values $[y, y+1, \ldots, y+25]$ (in order). For example, the value of `'e'` is $x+4$, and the value of `