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 5 5
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 100 2
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 100 3
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 1 2
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 2 2
nm
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 5 5
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 3 3
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 8 23
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 2 16
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kbub
 zbibx
 zq
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 helle
 hgn
 -1
 bacdef
 uy
 -1
 mn
 -1
 gyvnn
 -1
 vuh
 hx
 gc
 mgiomcytqdvoihhicrldmuj
 mnikno
 tvs
 -1
 -1
 -1
 -1

/**
 * DA-IICT
 * Author : PARTH PATEL
 */
import java.io.*;
import java.math.*;
import java.util.*;

import static java.util.Arrays.fill;
import static java.lang.Math.*;
import static java.util.Arrays.sort;
import static java.util.Collections.sort;


public class E903 {

	public static int mod = 1000000007;
	public static long INF = (1L << 60);
	static FasterScanner in = new FasterScanner();
	static PrintWriter out = new PrintWriter(System.out);
	public static String swapper(String s,int k1,int k2)
	{
		int m1=min(k1, k2);
		int m2=max(k1, k2);
		String answer=s.substring(0, m1)+s.charAt(m2)+s.substring(m1+1, m2)+s.charAt(m1)+s.substring(m2+1);
		return answer;
	}
	public static void main(String[] args) {

		int n=in.nextInt();
		int len=in.nextInt();
		String[] arr=new String[n+1];
		Set<String> set=new HashSet<>();
		for(int i=1;i<=n;i++)
		{
			arr[i]=in.nextLine();
			set.add(arr[i]);
		}
		if(set.size()==1)
		{
			System.out.println(arr[1].charAt(arr[1].length()-1)+""+arr[1].charAt(0)+arr[1].substring(1, arr[1].length()-1));
			return;
		}
		ArrayList<Integer>[][] diff=new ArrayList[n+1][n+1];
		for(int i=1;i<=n;i++)
		{
			for(int j=1;j<=n;j++)
			{
				diff[i][j]=new ArrayList<>();
				for(int s=0;s<len;s++)
				{
					if(arr[i].charAt(s)!=arr[j].charAt(s))
						diff[i][j].add(s);
				}
			}
		}
		for(int i=1;i<=n;i++)
		{
			for(int j=1;j<=n;j++)
			{
				if(diff[i][j].size()>4)
				{
					System.out.println("-1");
					return;
				}
			}
		}
		for(int i=1;i<=n;i++)
		{
			for(int j=1;j<=n;j++)
			{
				for(int k : diff[i][j])
				{
					//we have to swap kth character with other
					for(int ss=0;ss<len;ss++)
					{
						if(ss==k)
							continue;
						String temp=swapper(arr[i], k, ss);
						boolean alldistinct=true;
						Set<Character> characters=new HashSet<>();
						for(int x=0;x<temp.length();x++)
						{
							characters.add(temp.charAt(x));
						}
						alldistinct=(characters.size()==temp.length());
						//check if temp is valid for all strings
						boolean istemppossible=true;
						for(int str=1;str<=n;str++)
						{
							int diffcount=0;
							for(int cc=0;cc<len;cc++)
							{
								if(arr[str].charAt(cc)!=temp.charAt(cc))
									diffcount++;
							}
							if(diffcount==0 && !alldistinct)
								continue;
							if(diffcount==2)
								continue;
							istemppossible=false;
							break;
						}
						if(istemppossible)
						{
							System.out.println(temp);
							return;
						}
					}
				}
			}
		}
		out.println(-1);
		out.close();

	}

	public static long pow(long x, long n, long mod) {
		long res = 1;
		for (long p = x; n > 0; n >>= 1, p = (p * p) % mod) {
			if ((n & 1) != 0) {
				res = (res * p % mod);
			}
		}
		return res;
	}

	public static long gcd(long n1, long n2) {
		long r;
		while (n2 != 0) {
			r = n1 % n2;
			n1 = n2;
			n2 = r;
		}
		return n1;
	}

	public static long lcm(long n1, long n2) {
		long answer = (n1 * n2) / (gcd(n1, n2));
		return answer;
	}

	static class FasterScanner {
		private byte[] buf = new byte[1024];
		private int curChar;
		private int snumChars;

		public int read() {
			if (snumChars == -1)
				throw new InputMismatchException();
			if (curChar >= snumChars) {
				curChar = 0;
				try {
					snumChars = System.in.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (snumChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}

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

		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 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 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 < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}

		public int[] nextIntArray(int n) {
			int[] arr = new int[n];
			for (int i = 0; i < n; i++) {
				arr[i] = nextInt();
			}
			return arr;
		}

		public long[] nextLongArray(int n) {
			long[] arr = new long[n];
			for (int i = 0; i < n; i++) {
				arr[i] = nextLong();
			}
			return arr;
		}

		private boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
	}

}
