Spy-string

题意翻译

給你 $n$ 个长度为 $m$ 的字符串。 请你找到一个字符串：它和每个给定的字符串之间的差不超过 $1$。 給一下“差”的定义：两个长度相等的字符串中不同的字符的个数。（是逐位比较，不是LCS） translated by 90693

题目描述

You are given $ n $ strings $ a_1, a_2, \ldots, a_n $ : all of them have the same length $ m $ . The strings consist of lowercase English letters. Find any string $ s $ of length $ m $ such that each of the given $ n $ strings differs from $ s $ in at most one position. Formally, for each given string $ a_i $ , there is no more than one position $ j $ such that $ a_i[j] \ne s[j] $ . Note that the desired string $ s $ may be equal to one of the given strings $ a_i $ , or it may differ from all the given strings. For example, if you have the strings abac and zbab, then the answer to the problem might be the string abab, which differs from the first only by the last character, and from the second only by the first.

输入输出格式

输入格式

The first line contains an integer $ t $ ( $ 1 \le t \le 100 $ ) — the number of test cases. Then $ t $ test cases follow. Each test case starts with a line containing two positive integers $ n $ ( $ 1 \le n \le 10 $ ) and $ m $ ( $ 1 \le m \le 10 $ ) — the number of strings and their length. Then follow $ n $ strings $ a_i $ , one per line. Each of them has length $ m $ and consists of lowercase English letters.

输出格式

Print $ t $ answers to the test cases. Each answer (if it exists) is a string of length $ m $ consisting of lowercase English letters. If there are several answers, print any of them. If the answer does not exist, print "-1" ("minus one", without quotes).

输入输出样例

输入样例 #1

5
2 4
abac
zbab
2 4
aaaa
bbbb
3 3
baa
aaa
aab
2 2
ab
bb
3 1
a
b
c

输出样例 #1

abab
-1
aaa
ab
z

说明

The first test case was explained in the statement. In the second test case, the answer does not exist.