A11561. 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, $k·n<=5000$ ).

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.

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

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.