3 4
kbub
kbbu
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 5 5
zbibx
zbbix
zbibx
xbibz
xbibz
 100 2
qz
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qz
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qz
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qz
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 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
zsk
kzs
kzs
kzs
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zsk
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kzs
zsk
kzs
kzs
skz
 5 5
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ehlle
helle
hlele
 2 3
ghn
ghn
 2 4
ayax
abac
 2 6
abcdef
badcef
 1 2
yu
 2 4
ajax
azad
 2 2
nm
nm
 2 2
ac
ca
 5 5
gyvnn
gnvny
vygnn
gynvn
gnvny
 2 2
ab
cd
 3 3
uvh
vhu
huv
 3 2
xh
xh
xh
 3 2
cg
cg
cg
 8 23
mgiomcytqdvoihhcirldmuj
mgmoicytqdvoihhicrldmuj
mgmomcytqdvoihhicrldiuj
mgcomcytqdvoihhiirldmuj
mgiimcytqdvoihhocrldmuj
mgioicytqdvoihhmcrldmuj
mgiomcytqdvodhhicrlimuj
mgiomcytjdvoihhicrldmuq
 8 6
mnionk
mnikno
inmkno
mnnkio
oniknm
oniknm
mkinno
mnikon
 1 3
vts
 2 3
abc
abz
 2 4
abcd
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 2 5
dbcag
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 2 16
hmlqgaepsgpdbzyk
chhhburuvnyirrim

kbub
 zbibx
 zq
 szk
 helle
 hgn
 -1
 bacdef
 uy
 -1
 mn
 -1
 gyvnn
 -1
 vuh
 hx
 gc
 mgiomcytqdvoihhicrldmuj
 mnikno
 tvs
 -1
 -1
 -1
 -1

//package javaapplication1;
/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class Main {
    public static Set generatePos(String s){
    StringBuilder sb=new StringBuilder(s);
    char a,b;
    Set<String>Poss=new HashSet<>();
        for (int i = 0; i < s.length(); ++i) {
            for (int j = 0; j < s.length(); ++j) {
             sb=new StringBuilder(s);
             a=s.charAt(i);b=s.charAt(j);
             sb.setCharAt(i, b);
             sb.setCharAt(j, a);
             Poss.add(sb.toString());
            }
        }
    
    return Poss;
    }
    public static void main(String[] args) throws IOException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pw=new PrintWriter(new PrintStream(System.out));
        String [] input=br.readLine().split(" ");
        int k=Integer.parseInt(input[0]);
        int n=Integer.parseInt(input[1]);
        Set<String>ss=new HashSet<>();
        Set<String>possibilities=new HashSet<>();
        for (int i = 0; i < k; i++) {
            ss.add(br.readLine());
        }
        List<String>po=new ArrayList<>(ss);
        for(int i=0;i<po.size();++i){
            possibilities.addAll(generatePos(po.get(i)));
        }
        int c,cc;
        List<String>pos=new ArrayList<>(possibilities);
        for(int i=0;i<pos.size();++i){
            c=0;
            for (int j = 0; j < po.size(); ++j) {
                cc=0;
                for (int l = 0; l < Math.min(po.get(j).length(), pos.get(i).length()); ++l) {
                    if(po.get(j).charAt(l)!=pos.get(i).charAt(l))
                        ++cc;
                }
                if(cc==2||cc==0)
                    ++c;
            }
            if(c==po.size())
            {
                System.out.println(pos.get(i));
                System.exit(0);
            }
        }
        System.out.print(-1);
    }
    
}