We had a string s consisting of n lowercase Latin letters. We made k copies of this string, thus obtaining k identical strings s1, s2, ..., sk. After that, in each of these strings we swapped exactly two characters (the characters we swapped could be identical, but they had different indices in the string).

You are given k strings s1, s2, ..., sk, and you have to restore any string s so that it is possible to obtain these strings by performing aforementioned operations. Note that the total length of the strings you are given doesn't exceed 5000 (that is, k·n ≤ 5000).

Input

The first line contains two integers k and n (1 ≤ k ≤ 2500, 2 ≤ n ≤ 5000, k · n ≤ 5000) — the number of strings we obtained, and the length of each of these strings.

Next k lines contain the strings s1, s2, ..., sk, each consisting of exactly n lowercase Latin letters.

Output

Print any suitable string s, or -1 if such string doesn't exist.

Examples

Input

3 4
abac
caab
acba


Output

acab


Input

3 4
kbbu
kbub
ubkb


Output

kbub


Input

5 4
abcd
dcba
acbd
dbca
zzzz


Output

-1

Note

In the first example s1 is obtained by swapping the second and the fourth character in acab, s2 is obtained by swapping the first and the second character, and to get s3, we swap the third and the fourth character.

In the second example s1 is obtained by swapping the third and the fourth character in kbub, s2 — by swapping the second and the fourth, and s3 — by swapping the first and the third.

In the third example it's impossible to obtain given strings by aforementioned operations.
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.InputMismatchException;
import java.util.List;
import java.util.Map;

public class Main {

	private static final String NO = "NO";
	private static final String YES = "YES";
	InputStream is;
	PrintWriter out;
	String INPUT = "";
	private List<Integer>[] g;

	private static final long MOD = 1000000007;

	void solve() {
		int T = 1;
		for (int i = 0; i < T; i++)
			solve(i);
	}

	void solve(int T) {
		int n = ni();
		int k = ni();
		char s[][] = new char[n][];
		boolean twochars[] = new boolean[n];
		int mp[][] = new int[n][128];
		for (int i = 0; i < n; ++i) {
			s[i] = ns().toCharArray();
			for (char c : s[i])
				if (++mp[i][c] > 1)
					twochars[i] = true;
			if (!Arrays.equals(mp[i], mp[0])) {
				out.print(-1);
				return;
			}
		}
		int dis[] = new int[n];
		for (int i = 0; i < n; ++i)
			for (int j = 0; j < k; ++j)
				dis[i] += (s[0][j] != s[i][j] ? 1 : 0);
//		tr(dis);
		for (int x = 0; x < k; ++x)
			outer: for (int y = x + 1; y < k; ++y) {
				for (int i = 1; i < n; ++i)
					if (dis[i] - (s[i][x] != s[0][x] ? 1 : 0) + (s[i][x] != s[0][y] ? 1 : 0)
							- (s[i][y] != s[0][y] ? 1 : 0) + (s[i][y] != s[0][x] ? 1 : 0) > 2
							|| dis[i] - (s[i][x] != s[0][x] ? 1 : 0) + (s[i][x] != s[0][y] ? 1 : 0)
									- (s[i][y] != s[0][y] ? 1 : 0) + (s[i][y] != s[0][x] ? 1 : 0) == 0 && !twochars[i])
						continue outer;
				char tmp = s[0][x];
				s[0][x] = s[0][y];
				s[0][y] = tmp;
				out.print(new String(s[0]));
				return;
			}
		out.print("-1");
	}

	// a^b
	long power(long a, long b) {
		long x = 1, y = a;
		while (b > 0) {
			if (b % 2 != 0) {
				x = (x * y) % MOD;
			}
			y = (y * y) % MOD;
			b /= 2;
		}
		return x % MOD;
	}

	private long gcd(long a, long b) {
		while (a != 0) {
			long tmp = b % a;
			b = a;
			a = tmp;
		}
		return b;
	}

	void run() throws Exception {
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);

		long s = System.currentTimeMillis();
		solve();
		out.flush();
		if (!INPUT.isEmpty())
			tr(System.currentTimeMillis() - s + "ms");
	}

	public static void main(String[] args) throws Exception {
		new Main().run();
	}

	private byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;
	private boolean vis[];

	private int readByte() {
		if (lenbuf == -1)
			throw new InputMismatchException();
		if (ptrbuf >= lenbuf) {
			ptrbuf = 0;
			try {
				lenbuf = is.read(inbuf);
			} catch (IOException e) {
				throw new InputMismatchException();
			}
			if (lenbuf <= 0)
				return -1;
		}
		return inbuf[ptrbuf++];
	}

	private boolean isSpaceChar(int c) {
		return !(c >= 33 && c <= 126);
	}

	private int skip() {
		int b;
		while ((b = readByte()) != -1 && isSpaceChar(b))
			;
		return b;
	}

	private double nd() {
		return Double.parseDouble(ns());
	}

	private char nc() {
		return (char) skip();
	}

	private String ns() {
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while (!(isSpaceChar(b))) { // when nextLine, (isSpaceChar(b) && b != '
									// ')
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	private char[] ns(int n) {
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while (p < n) {
			if (!(isSpaceChar(b)))
				buf[p++] = (char) b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}

	private char[][] nm(int n, int m) {
		char[][] map = new char[n][];
		for (int i = 0; i < n; i++)
			map[i] = ns(m);
		return map;
	}

	private int[] na(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = ni();
		return a;
	}

	private List<Integer> na2(int n) {
		List<Integer> a = new ArrayList<Integer>();
		for (int i = 0; i < n; i++)
			a.add(ni());
		return a;
	}

	private int[][] na(int n, int m) {
		int[][] a = new int[n][];
		for (int i = 0; i < n; i++)
			a[i] = na(m);
		return a;
	}

	private int ni() {
		int num = 0, b;
		boolean minus = false;
		while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
			;
		if (b == '-') {
			minus = true;
			b = readByte();
		}

		while (true) {
			if (b >= '0' && b <= '9') {
				num = num * 10 + (b - '0');
			} else {
				return minus ? -num : num;
			}
			b = readByte();
		}
	}

	private long[] nl(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)
			a[i] = nl();
		return a;
	}

	private long[][] nl(int n, int m) {
		long[][] a = new long[n][];
		for (int i = 0; i < n; i++)
			a[i] = nl(m);
		return a;
	}

	private long nl() {
		long num = 0;
		int b;
		boolean minus = false;
		while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
			;
		if (b == '-') {
			minus = true;
			b = readByte();
		}

		while (true) {
			if (b >= '0' && b <= '9') {
				num = num * 10 + (b - '0');
			} else {
				return minus ? -num : num;
			}
			b = readByte();
		}
	}

	private static void tr(Object... o) {
		System.out.println(Arrays.deepToString(o));
	}

	public class Pair<K, V> {

		/**
		 * Key of this <code>Pair</code>.
		 */
		private K key;

		/**
		 * Gets the key for this pair.
		 * 
		 * @return key for this pair
		 */
		public K getKey() {
			return key;
		}

		/**
		 * Value of this this <code>Pair</code>.
		 */
		private V value;

		/**
		 * Gets the value for this pair.
		 * 
		 * @return value for this pair
		 */
		public V getValue() {
			return value;
		}

		/**
		 * Creates a new pair
		 * 
		 * @param key   The key for this pair
		 * @param value The value to use for this pair
		 */
		public Pair(K key, V value) {
			this.key = key;
			this.value = value;
		}

		/**
		 * <p>
		 * <code>String</code> representation of this <code>Pair</code>.
		 * </p>
		 *
		 * <p>
		 * The default name/value delimiter '=' is always used.
		 * </p>
		 *
		 * @return <code>String</code> representation of this <code>Pair</code>
		 */
		@Override
		public String toString() {
			return key + "=" + value;
		}

		/**
		 * <p>
		 * Generate a hash code for this <code>Pair</code>.
		 * </p>
		 *
		 * <p>
		 * The hash code is calculated using both the name and the value of the
		 * <code>Pair</code>.
		 * </p>
		 *
		 * @return hash code for this <code>Pair</code>
		 */
		@Override
		public int hashCode() {
			// name's hashCode is multiplied by an arbitrary prime number (13)
			// in order to make sure there is a difference in the hashCode between
			// these two parameters:
			// name: a value: aa
			// name: aa value: a
			return key.hashCode() * 13 + (value == null ? 0 : value.hashCode());
		}

		/**
		 * <p>
		 * Test this <code>Pair</code> for equality with another <code>Object</code>.
		 * </p>
		 *
		 * <p>
		 * If the <code>Object</code> to be tested is not a <code>Pair</code> or is
		 * <code>null</code>, then this method returns <code>false</code>.
		 * </p>
		 *
		 * <p>
		 * Two <code>Pair</code>s are considered equal if and only if both the names and
		 * values are equal.
		 * </p>
		 *
		 * @param o the <code>Object</code> to test for equality with this
		 *          <code>Pair</code>
		 * @return <code>true</code> if the given <code>Object</code> is equal to this
		 *         <code>Pair</code> else <code>false</code>
		 */
		@Override
		public boolean equals(Object o) {
			if (this == o)
				return true;
			if (o instanceof Pair) {
				Pair pair = (Pair) o;
				if (key != null ? !key.equals(pair.key) : pair.key != null)
					return false;
				if (value != null ? !value.equals(pair.value) : pair.value != null)
					return false;
				return true;
			}
			return false;
		}
	}

}