CF801B.Valued Keys

You found a mysterious function f. The function takes two strings _s_1 and _s_2. These strings must consist only of lowercase English letters, and must be the same length.

The output of the function f is another string of the same length. The i-th character of the output is equal to the minimum of the i-th character of _s_1 and the i-th character of _s_2.

For example, f("ab", "ba") = "aa", and f("nzwzl", "zizez") = "niwel".

You found two strings x and y of the same length and consisting of only lowercase English letters. Find any string z such that f(x, z) = y, or print -1 if no such string z exists.

你发现了一个神秘的函数 $f$。该函数接收两个字符串 $s_1$ 和 $s_2$，这两个字符串必须仅由小写英文字母组成，且长度相同。

函数 $f$ 的输出是另一个等长的字符串；其第 $i$ 个字符等于 $s_1$ 的第 $i$ 个字符与 $s_2$ 的第 $i$ 个字符中的较小者（按字母表顺序）。

例如，$f$("ab", "ba") = "aa"，且 $f$("nzwzl", "zizez") = "niwel"。

你找到了两个等长的字符串 $x$ 和 $y$，它们均由小写英文字母组成。请找出任意一个字符串 $z$，使得 $f(x,\,z) = y$；若不存在这样的字符串 $z$，则输出 $-1$。

The first line of input contains the string x.

The second line of input contains the string y.

Both x and y consist only of lowercase English letters, x and y have same length and this length is between 1 and 100.

输入的第一行包含字符串 $x$。

输入的第二行包含字符串 $y$。

字符串 $x$ 和 $y$ 均仅由小写英文字母组成，且长度相同，该长度在 $1$ 到 $100$ 之间。

If there is no string z such that f(x, z) = y, print -1.

Otherwise, print a string z such that f(x, z) = y. If there are multiple possible answers, print any of them. The string z should be the same length as x and y and consist only of lowercase English letters.

如果不存在字符串 $z$，使得 $f(x, z) = y$，则输出 -1。

否则，输出一个满足 $f(x, z) = y$ 的字符串 $z$。若存在多个可能的答案，输出任意一个即可。字符串 $z$ 的长度应与 $x$ 和 $y$ 相同，且仅由小写英文字母组成。

The first case is from the statement.

Another solution for the second case is "zizez"

There is no solution for the third case. That is, there is no z such that f("ab", z) =  "ba".

第一种情况来自题目描述。

第二种情况的另一种解是 “zizez”。

第三种情况无解。即，不存在满足 $f(\text{"ab"}, z) = \text{"ba"}$ 的字符串 $z$。