CF903E.Swapping Characters

We had a string s consisting of n lowercase Latin letters. We made k copies of this string, thus obtaining k identical strings _s_1, _s_2, ..., s__k. After that, in each of these strings we swapped exactly two characters (the characters we swapped could be identical, but they had different indices in the string).

You are given k strings _s_1, _s_2, ..., s__k, and you have to restore any string s so that it is possible to obtain these strings by performing aforementioned operations. Note that the total length of the strings you are given doesn't exceed 5000 (that is, k·n ≤ 5000).

我们有一个由 $n$ 个小写拉丁字母组成的字符串 $s$。我们制作了该字符串的 $k$ 个副本，从而得到 $k$ 个完全相同的字符串 $s_1,\,s_2,\,\dots,\,s_k$。随后，在每个字符串中，我们恰好交换了两个字符（被交换的字符可以相同，但它们在字符串中的下标必须不同）。

你将收到 $k$ 个字符串 $s_1,\,s_2,\,\dots,\,s_k$，你需要还原出任意一个字符串 $s$，使得通过上述操作可以得到这些字符串。注意：你所收到的所有字符串的总长度不超过 5000（即 $k \cdot n \leq 5000$）。

The first line contains two integers k and n (1 ≤ k ≤ 2500, 2 ≤ n ≤ 5000, k · n ≤ 5000) — the number of strings we obtained, and the length of each of these strings.

Next k lines contain the strings _s_1, _s_2, ..., s__k, each consisting of exactly n lowercase Latin letters.

第一行包含两个整数 $k$ 和 $n$（$1 \leq k \leq 2500$，$2 \leq n \leq 5000$，且 $k \cdot n \leq 5000$）—— 分别表示我们获得的字符串数量，以及每个字符串的长度。

接下来的 $k$ 行包含字符串 $s_1,\,s_2,\,...,\,s_k$，每个字符串均由恰好 $n$ 个小写拉丁字母组成。

Print any suitable string s, or -1 if such string doesn't exist.

输出任意一个符合条件的字符串 $s$，如果不存在这样的字符串，则输出 $-1$。

In the first example _s_1 is obtained by swapping the second and the fourth character in acab, _s_2 is obtained by swapping the first and the second character, and to get _s_3, we swap the third and the fourth character.

In the second example _s_1 is obtained by swapping the third and the fourth character in kbub, _s_2 — by swapping the second and the fourth, and _s_3 — by swapping the first and the third.

In the third example it's impossible to obtain given strings by aforementioned operations.

在第一个例子中，$s_1$ 是通过对字符串 acab 的第二个和第四个字符进行交换得到的，$s_2$ 是通过对第一个和第二个字符进行交换得到的，而 $s_3$ 则是通过对第三个和第四个字符进行交换得到的。

在第二个例子中，$s_1$ 是通过对字符串 kbub 的第三个和第四个字符进行交换得到的，$s_2$ 是通过对第二个和第四个字符进行交换得到的，$s_3$ 是通过对第一个和第三个字符进行交换得到的。

在第三个例子中，无法通过上述操作得到所给的字符串。