题目描述

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.

输入格式

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 $100$ .

输出格式

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.

题目大意

找出来一个字符串s3，能让s1的第i位跟s3的第i位比较，二者最小的是s2的第i位 大小比较方法为比较字典序大小 Translated by @liyifeng

ab
aa

ba

nzwzl
niwel

xiyez

ab
ba

-1

提示

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".