3 4
kbub
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 5 5
zbibx
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 100 2
qz
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qz
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qz
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qz
qz
 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
zsk
kzs
kzs
kzs
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 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
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mkinno
mnikon
 1 3
vts
 2 3
abc
abz
 2 4
abcd
ccdc
 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

/*
 * PDPM IIITDM Jabalpur
 * Asutosh Rana
 */

import java.util.*;
import java.io.*;
import java.math.*;

public class Main
{
    long MOD = 1000000007;
    InputReader in;BufferedReader br;PrintWriter out;
    public static void main (String[] args) throws java.lang.Exception
    {
        Main solver = new Main();
        solver.in = new InputReader(System.in);
        solver.br = new BufferedReader(new InputStreamReader(System.in));
        solver.out = new PrintWriter(System.out);
        solver.solve();
        solver.out.flush();
        solver.out.close();
    }

    public void solve(){
        
        int tc = 1;//in.readInt();
        
        for(int cas=1;cas<=tc;cas++){
            int K = in.readInt();
            int N = in.readInt();
            
            char[][] S = new char[K][];
            for(int i=0;i<K;i++){
                S[i] = in.readCharA();
            }

            int[][] C = new int[K][N];
            for(int i=1;i<K;i++){
                for(int j=0;j<N;j++){
                    C[i][j] = (S[i][j]==S[0][j])?0:1;
                }
            }

            for(int i=0;i<K;i++){
                for(int j=1;j<N;j++){
                    C[i][j] += C[i][j-1];
                }
            }

            boolean cb0 = false;
            int[] D = new int[26];
            for(int i=0;i<N;i++){
                D[S[0][i]-'a']++;
                if(D[S[0][i]-'a']>1)
                    cb0=true;
            }


            int done = 0;
            for(int i=0;i<N && done==0;i++){
                for(int j=i+1;j<N && done==0;j++){
                    int flag = 0;
                    for(int p=0;p<K;p++){
                        int diff = (i>0?C[p][i-1]:0) +(C[p][j-1] - C[p][i])+(C[p][N-1]-C[p][j]);
                        diff += ((S[0][i]==S[p][j])?0:1) + ((S[0][j]==S[p][i])?0:1);
                        if(diff>2){
                            flag=1;break;
                        }
                        else if(diff==0 && cb0==false){
                            flag=1;break;
                        }
                    }
                    if(flag==0){
                        print(S[0], i, j); done++;break;
                    }
                }
            }

            if(done==0){
                out.println("-1");
            }
        }

    }

    void print(char[] S, int i , int j){
        for(int k=0;k<S.length;k++){
            if(k==i){
                out.print(S[j]);
            }
            else if(k==j){
                out.print(S[i]);
            }
            else
                out.print(S[k]);
        }
    }
    
}
 
class InputReader{
    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;

    public InputReader(InputStream stream){this.stream = stream;}
    public int read(){
        if (numChars==-1) throw new InputMismatchException();
        if (curChar >= numChars){
            curChar = 0;
            try {numChars = stream.read(buf);}
            catch (IOException e){throw new InputMismatchException();}
            if(numChars <= 0) return -1;
        }
        return buf[curChar++];
    }

    public int readInt(){
        int c = read();
        while(isSpaceChar(c)) c = read();
        int sgn = 1;
        if (c == '-') {sgn = -1;c = read();}
        int res = 0;
        do {
            if(c<'0'||c>'9') throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        }
        while (!isSpaceChar(c)); return res * sgn;
    }

    public void readInt(int[] A){
        for(int i=0;i<A.length;i++)
            A[i] = readInt();
    }

    public long readLong() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        long res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        }
        while (!isSpaceChar(c));
        return res * sgn;
    }

    public void readLong(long[] A){
        for(int i=0;i<A.length;i++)
            A[i] = readLong();
    }

    public double readDouble() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        double res = 0;
        while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E')
                return res * Math.pow(10, readInt());
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        }
        if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, readInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                m /= 10;
                res += (c - '0') * m;
                c = read();
            }
        }
        return res * sgn;
    }

    public char[] readCharA(){
        return readString().toCharArray();
    }

    public String readString() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public boolean isSpaceChar(int c) {
        if (filter != null)
            return filter.isSpaceChar(c);
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    public String next() {
        return readString();
    }

    public interface SpaceCharFilter {
        public boolean isSpaceChar(int ch);
    }

}