3 4
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 5 5
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 100 2
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 100 3
zsk
zsk
zsk
zsk
skz
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kzs
skz
kzs
kzs
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kzs
kzs
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 5 5
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 2 3
ghn
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 2 4
ayax
abac
 2 6
abcdef
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 1 2
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 2 4
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 2 2
nm
nm
 2 2
ac
ca
 5 5
gyvnn
gnvny
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gynvn
gnvny
 2 2
ab
cd
 3 3
uvh
vhu
huv
 3 2
xh
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 3 2
cg
cg
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 8 23
mgiomcytqdvoihhcirldmuj
mgmoicytqdvoihhicrldmuj
mgmomcytqdvoihhicrldiuj
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mgiimcytqdvoihhocrldmuj
mgioicytqdvoihhmcrldmuj
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mgiomcytjdvoihhicrldmuq
 8 6
mnionk
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 1 3
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 2 3
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 2 16
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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.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.HashSet;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskE solver = new TaskE();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskE {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int k = in.readInt();
            int n = in.readInt();
            char[][] str = new char[k][n];
            for (int i = 0; i < k; i++) {
                str[i] = in.readLine().toCharArray();
            }
            boolean dup = false;
            char[] ts = str[0].clone();
            Arrays.sort(ts);
            for (int i = 1; i < n; i++) if (ts[i] == ts[i - 1]) dup = true;

            HashSet<Integer> mismatch = new HashSet<>();
            for (int j = 1; j < k; j++) {
                for (int i = 0; i < n; i++) {
                    if (str[0][i] != str[j][i]) {
                        mismatch.add(i);
                    }
                }
                if (mismatch.size() > 0) break;
            }
            if (mismatch.size() == 0) {
                char[] res = str[0];
                char t = res[0];
                res[0] = res[1];
                res[1] = t;
                out.println(new String(res));
                return;
            }

            if (mismatch.size() > 4) {
                out.println(-1);
                return;
            }
            ArrayList<Integer> list = new ArrayList<>(mismatch);
            for (int i = 0; i < list.size(); i++) {
                for (int j = 0; j < n; j++) {
                    int l = list.get(i), r = j;
                    if (l == r) continue;
                    char[] temp = str[0].clone();
                    char t = temp[l];
                    temp[l] = temp[r];
                    temp[r] = t;
                    boolean valid = true;
                    for (int p = 0; p < k; p++) {
                        if (!valid(str[p], temp, dup)) {
                            valid = false;
                            break;
                        }
                    }
                    if (valid) {
                        out.println(new String(temp));
                        return;
                    }
                }
            }
            out.println(-1);
        }

        boolean valid(char[] a, char[] b, boolean dup) {
            ArrayList<Integer> mismatch = new ArrayList<>();
            for (int i = 0; i < a.length; i++) {
                if (a[i] != b[i]) mismatch.add(i);
            }
            if (mismatch.size() == 0 && !dup) return false;
            if (mismatch.size() == 0) return true;
            if (mismatch.size() != 2) return false;
            if (a[mismatch.get(0)] == b[mismatch.get(1)] && a[mismatch.get(1)] == b[mismatch.get(0)]) return true;
            return false;
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.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 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 readLine() {
            String s = readLine0();
            while (s.trim().length() == 0)
                s = readLine0();
            return s;
        }

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

        }

    }
}

