3 4
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 5 5
zbibx
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 100 2
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 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
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kzs
kzs
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 5 5
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ghn
ghn
 2 4
ayax
abac
 2 6
abcdef
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 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
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 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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 1 3
vts
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abc
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abcd
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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

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

public class e {


    public static void main(String[] args) throws IOException {
        FastScanner in = new FastScanner(System.in);
        PrintWriter out = new PrintWriter(System.out);
        
        int n = in.nextInt();
        int k = in.nextInt();
        
        char[][] strs = new char[n][];
        for (int i = 0; i < n; i++) {
            strs[i] = in.next().toCharArray();
        }
        
        IntList[] indices = new IntList[n];
        boolean[] hasRepeat = new boolean[n];
        int max = 0;
        for (int i = 1; i < n; i++) {
            boolean[] has = new boolean[26];
            indices[i] = new IntList();
            for (int j = 0; j < k; j++) {
                if (strs[0][j] != strs[i][j]) {
                    indices[i].add(j);
                }
                if (has[strs[i][j] - 'a']) {
                    hasRepeat[i] = true;
                }
                has[strs[i][j] - 'a'] = true;
            }
            max = Math.max(max, indices[i].size());
        }
        
        if (max > 4) {
            out.println(-1);
            out.close();
            return;
        }
        
        for (int ai = 0; ai < k; ai++) {
            for (int bi = ai + 1; bi < k; bi++) {
                char t = strs[0][ai];
                strs[0][ai] = strs[0][bi];
                strs[0][bi] = t;
                
                boolean good = true;
                
                for (int i = 1; i < n; i++) {
                    IntList inds = indices[i];
                    if (inds.size() == 0) continue;
                    inds.add(ai);
                    inds.add(bi);
                    
                    if (hasRepeat[i] && check(strs[0], strs[i], inds)) {
                        inds.n--;
                        inds.n--;
                        continue;
                    }
                    boolean good2 = false;
outer: 
                    for (int cii = 0; cii < inds.size(); cii++) {
                        for (int dii = cii; dii < inds.size(); dii++) {
                            int ci = inds.get(cii);
                            int di = inds.get(dii);
                            if (ci == di) continue;
                            t = strs[i][ci];
                            strs[i][ci] = strs[i][di];
                            strs[i][di] = t;
                            
                            boolean tmp = check(strs[0], strs[i], inds);
                            
                            t = strs[i][ci];
                            strs[i][ci] = strs[i][di];
                            strs[i][di] = t;
                            
                            if (tmp) {
                                good2 = true;
                                break outer;
                            }
                        }
                    }
                    
                    inds.n--;
                    inds.n--;
                    if (!good2) {
                        good = false;
                        break;
                    }
                }
                
                if (good) {
                    out.println(new String(strs[0]));
                    out.close();
                    return;
                } else {
                    t = strs[0][ai];
                    strs[0][ai] = strs[0][bi];
                    strs[0][bi] = t;
                }
            }
        }
        
        
        out.println(-1);
        
        out.close();
    }


    static boolean check(char[] a, char[] b, IntList inds) {
        for (int i = 0; i < inds.size(); i++) {
            int j = inds.get(i);
            if (a[j] != b[j]) return false;
        }
        return true;
    }
    static Random rand = new Random();
    static void sort(int[] a) {
        int n = a.length;
        for (int i = a.length - 1; i > 0; i--) {
            int j = rand.nextInt(i + 1);
            int tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }
        Arrays.sort(a);
    }
    static void sort(long[] a) {
        int n = a.length;
        for (int i = a.length - 1; i > 0; i--) {
            int j = rand.nextInt(i + 1);
            long tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }
        Arrays.sort(a);
    }
    static void sort(double[] a) {
        int n = a.length;
        for (int i = a.length - 1; i > 0; i--) {
            int j = rand.nextInt(i + 1);
            double tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }
        Arrays.sort(a);
    }
    static long gcd(long a, long b) { return b == 0 ? a : gcd(b, a % b); }
    static long lcm(long a, long b) { return a / gcd(a, b) * b; }
    static long[] eEuclid(long a, long b) {
        if (b == 0) return new long[] { a, 1, 0 };
        long[] ans = eEuclid(b, a % b);
        long temp = ans[1] - ans[2] * (a / b);
        ans[1] = ans[2];  ans[2] = temp;
        return ans;
    }
    static long modInverse(long a, long m) {
        return ((eEuclid(a, m)[1] % m) + m) % m;
    }
    static class IntList {
        static int[] EMPTY = {};
        int[] a = EMPTY;
        int n = 0;
        void add(int v) {
            if (n >= a.length)
                a = Arrays.copyOf(a, (n << 2) + 8);
            a[n++] = v;
        }
        int get(int idx) {
            return a[idx];
        }
        int size() {
            return n;
        }
    }
    static class DisjointSet {
        int[] s, r;	
	public DisjointSet(int n) {
            s = new int[n]; r = new int[n];
            for (int i = 0; i < n; i++) s[i] = i;
        }	
        public int find(int i) { return s[i] == i ? i : (s[i] = find(s[i])); }
	public void union(int a, int b) {
            if(r[a = find(a)] == r[b = find(b)]) r[a]++;
            if(r[a] >= r[b]) s[b] = a; else s[a] = b;
        }
    }
    static class FastScanner {
        BufferedReader br;
        StringTokenizer st;
        public FastScanner(InputStream i) {
            br = new BufferedReader(new InputStreamReader(i));
            st = new StringTokenizer("");
        }
        public String next() throws IOException {
            if(st.hasMoreTokens())
                return st.nextToken();
            else
                st = new StringTokenizer(br.readLine());
            return next();
        }
        public int nextInt() throws IOException {
            return Integer.parseInt(next());
        }
        public long nextLong() throws IOException {
            return Long.parseLong(next());
        }
        public double nextDouble() throws IOException {
            return Double.parseDouble(next());
        }
        public int[] nextOffsetIntArray(int n) throws IOException {
            int[] arr = new int[n];
            for (int i = 0; i < n; i++)
                arr[i] = nextInt() - 1;
            return arr;
        }
        public int[] nextIntArray(int n) throws IOException {
            int[] arr = new int[n];
            for (int i = 0; i < n; i++)
                arr[i] = nextInt();
            return arr;
        }
        public int[][] nextIntArray(int n, int m) throws IOException {
            int[][] arr = new int[n][m];
            for (int i = 0; i < n; i++)
                for (int j = 0; j < m; j++)
                    arr[i][j] = nextInt();
            return arr;
        }
        public long[] nextLongArray(int n) throws IOException {
            long[] arr = new long[n];
            for (int i = 0; i < n; i++)
                arr[i] = nextLong();
            return arr;
        }
        public long[][] nextLongArray(int n, int m) throws IOException {
            long[][] arr = new long[n][m];
            for (int i = 0; i < n; i++)
                for (int j = 0; j < m; j++)
                    arr[i][j] = nextLong();
            return arr;
        }
        public double[] nextDoubleArray(int n) throws IOException {
            double[] arr = new double[n];
            for (int i = 0; i < n; i++)
                arr[i] = nextDouble();
            return arr;
        }
        public double[][] nextDoubleArray(int n, int m) throws IOException {
            double[][] arr = new double[n][m];
            for (int i = 0; i < n; i++)
                for (int j = 0; j < m; j++)
                    arr[i][j] = nextDouble();
            return arr;
        }
        public char[][] nextCharArray(int n, int m) throws IOException {
            char[][] arr = new char[n][];
            for (int i = 0; i < n; i++)
                arr[i] = next().toCharArray();
            return arr;
        }
    }
}
