3 4
kbub
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 5 5
zbibx
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 100 2
qz
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 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
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kzs
kzs
kzs
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 5 5
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hlele
 2 3
ghn
ghn
 2 4
ayax
abac
 2 6
abcdef
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 1 2
yu
 2 4
ajax
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 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
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 1 3
vts
 2 3
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.*;
import java.lang.*;

public class Main {
    private static FastReader sc = new FastReader(System.in);
    private static OutputWriter out = new OutputWriter(System.out);

    public static void main(String[] args) throws Exception {
        int k = sc.nextInt();
        int n = sc.nextInt();
        String[] strings = new String[k];
        for (int i = 0; i < k; i++) strings[i] = sc.next();

        Trie trie = new Trie(200);

        for (int i = 0; i < k; i++) {
            char[] arr = strings[i].toCharArray();
            for (int p = 0; p < n; p++) {
                for (int q = p + 1; q < n; q++) {
                    char ch1 = strings[i].charAt(p);
                    char ch2 = strings[i].charAt(q);
                    arr[p] = ch2;
                    arr[q] = ch1;
                    trie.put(arr, i);
                    arr[p] = ch1;
                    arr[q] = ch2;
                }
            }
        }


        ArrayList<Character> l = trie.getAns(n, k);
        if (l.isEmpty()) out.printLine(-1);
        else {
            for (int i = 1; i < l.size(); i++) out.print(l.get(i));
        }
        out.close();
    }

}

class Trie {
    int R;
    Node root;
    boolean ansExists;
    ArrayList<Character> sb;
    ArrayList<Character> ansString;

    public Trie(int r) {
        this.R = r;
        root = new Node();
        ansExists = false;
        sb = new ArrayList<>();
        ansString = new ArrayList<>();
    }

    public class Node {
        Node[] next = new Node[R];
        HashSet<Integer> set;
        char ch;
    }

    public ArrayList<Character> getAns(int n, int k) {
        getAnsUtil(root, 0, n, k);
        if (ansString != null) return ansString;
        else return new ArrayList<>();
    }

    private void getAnsUtil(Node x, int d, int n, int k) {
        if (x == null) return;
        if (d == n) {
            if (x.set != null && x.set.size() >= k) {
                ansExists = true;
                sb.add(x.ch);
                ansString = new ArrayList<>(sb);
            }
            return;
        }

        sb.add(x.ch);
        for (Node nex : x.next) {
            getAnsUtil(nex, d + 1, n, k);
        }
        sb.remove(sb.size() - 1);
    }

    public void put(char[] key, int idx) {
        root = putUtil(root, key, 0, idx);
    }

    private Node putUtil(Node x, char[] key, int d, int idx) {
        if (x == null) x = new Node();
        if (d == key.length) {
            if (x.set == null) x.set = new HashSet<>();
            x.set.add(idx);
            x.ch = key[d - 1];
            return x;
        }
        int ch = Character.getNumericValue(key[d]);
        x.next[ch] = putUtil(x.next[ch], key, d + 1, idx);
        x.next[ch].ch = key[d];
        return x;
    }
}

class FastReader {
    private boolean finished = false;

    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;

    public FastReader(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 peek() {
        if (numChars == -1) {
            return -1;
        }
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                return -1;
            }
            if (numChars <= 0) {
                return -1;
            }
        }
        return buf[curChar];
    }

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

    public long nextLong() {
        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 String nextString() {
        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 isWhitespace(c);
    }

    public static boolean isWhitespace(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    private String readLine0() {
        StringBuilder buf = new StringBuilder();
        int c = read();
        while (c != '\n' && c != -1) {
            if (c != '\r') {
                buf.appendCodePoint(c);
            }
            c = read();
        }
        return buf.toString();
    }

    public String nextLine() {
        String s = readLine0();
        while (s.trim().length() == 0)
            s = readLine0();
        return s;
    }

    public String nextLine(boolean ignoreEmptyLines) {
        if (ignoreEmptyLines) {
            return nextLine();
        } else {
            return readLine0();
        }
    }

    public BigInteger nextBigInteger() {
        try {
            return new BigInteger(nextString());
        } catch (NumberFormatException e) {
            throw new InputMismatchException();
        }
    }

    public char nextCharacter() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        return (char) c;
    }

    public double nextDouble() {
        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, nextInt());
            }
            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, nextInt());
                }
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                m /= 10;
                res += (c - '0') * m;
                c = read();
            }
        }
        return res * sgn;
    }

    public boolean isExhausted() {
        int value;
        while (isSpaceChar(value = peek()) && value != -1)
            read();
        return value == -1;
    }

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

    public SpaceCharFilter getFilter() {
        return filter;
    }

    public void setFilter(SpaceCharFilter filter) {
        this.filter = filter;
    }

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

class OutputWriter {
    private final PrintWriter writer;

    public OutputWriter(OutputStream outputStream) {
        writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
    }

    public OutputWriter(Writer writer) {
        this.writer = new PrintWriter(writer);
    }

    public void print(Object... objects) {
        for (int i = 0; i < objects.length; i++) {
            if (i != 0)
                writer.print(' ');
            writer.print(objects[i]);
        }
    }

    public void printLine(Object... objects) {
        print(objects);
        writer.println();
    }

    public void close() {
        writer.close();
    }

    public void flush() {
        writer.flush();
    }

}