P3742 umi's Function

umi found a mysterious function $f$.

This function takes two strings $s1, s2$. These strings consist only of lowercase letters and have the same length. The output of this function is another string $g$ whose length is the same as $s1$ and $s2$. The $i$-th character of $g$ equals the minimum of the $i$-th character of $s1$ and $s2$; simply put, $g_i=\min(s1_i,s2_i)$. For example: $f(\verb!ab!,\verb!ba!)= \verb!aa!$, $f(\verb!nzwzl!,\verb!zizez!)=\verb!niwel!$. She now has two strings $x, y$ of the same length, consisting only of lowercase letters. Find any string $z$ such that $f(x,z)=y$. If no such string exists, please output $-1$.

The first line contains the length $n$ of the two strings. The second line contains the string $x$. The third line contains the string $y$.

Output one string in the first line, representing a valid string you found. If none exists, output $-1$.

Constraints: For $100\%$ of the testdata, $1 \le n \le 100$. Translated by ChatGPT 5