3 4
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 5 5
zbibx
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 100 2
qz
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qz
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qz
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qz
qz
 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
zsk
kzs
kzs
kzs
skz
kzs
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kzs
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kzs
kzs
skz
 5 5
eellh
ehlle
ehlle
helle
hlele
 2 3
ghn
ghn
 2 4
ayax
abac
 2 6
abcdef
badcef
 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
xh
 3 2
cg
cg
cg
 8 23
mgiomcytqdvoihhcirldmuj
mgmoicytqdvoihhicrldmuj
mgmomcytqdvoihhicrldiuj
mgcomcytqdvoihhiirldmuj
mgiimcytqdvoihhocrldmuj
mgioicytqdvoihhmcrldmuj
mgiomcytqdvodhhicrlimuj
mgiomcytjdvoihhicrldmuq
 8 6
mnionk
mnikno
inmkno
mnnkio
oniknm
oniknm
mkinno
mnikon
 1 3
vts
 2 3
abc
abz
 2 4
abcd
ccdc
 2 5
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

#include <bits/stdc++.h>
using namespace std;
double const EPS = 1.0E-9;
int const MOD = (int)1e9 + 7;
inline int read(char* s) { return scanf("%s", s); }
inline int read(int& x) { return scanf("%d", &(x)); }
inline int read(long long& x) { return scanf("%lld", &x); }
inline int read(double& x) { return scanf("%lf", &x); }
inline int writeln(int x) { return printf("%d\n", x); }
inline int writeln(long long x) { return printf("%lld\n", x); }
inline int writeln(double x) { return printf("%.12f\n", x); }
template <typename... T>
int read(T&... args) {
  using swallow = int[];
  swallow w = {0, (read(args))...};
  return sizeof(w) / sizeof(w[0]) - 1;
}
inline long long gcd(long long x, long long y) {
  return y == 0 ? x : gcd(y, x % y);
}
inline long long lcm(long long x, long long y) {
  return x == 0 && y == 0 ? 0 : x / gcd(x, y) * y;
}
inline long long powmod(long long x, long long n, long long m = MOD) {
  long long r = 1;
  while (n) {
    if (n & 1) r = (r * x) % m;
    x = (x * x) % m;
    n /= 2;
  }
  return r;
}
static int leap(int y) { return y % 4 == 0 && y % 100 != 0 || y % 400 == 0; }
static int const month[2][12] = {
    {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}};
static const int N = 1 << 18;
static const int M = 1 << 20;
char s[2508][5000];
int n, k;
static int solve() {
  read(k, n);
  for (int i = (0), vzq_b = (k); i < vzq_b; ++i) {
    read(s[i]);
  }
  bool has_dif = false;
  for (int i = 0; i < k; ++i) {
    for (int j = i + 1; j < k; ++j) {
      std::vector<int> dif;
      for (int l = 0; l < n; ++l) {
        if (s[i][l] != s[j][l]) {
          dif.push_back(l);
          if (dif.size() > 4) {
            return puts("-1");
          }
        }
      }
      if (dif.empty()) continue;
      has_dif = true;
      for (int t = 0; t < (int)dif.size(); ++t) {
        for (int h = t + 1; h < (int)dif.size(); ++h) {
          int D = 2504;
          memcpy(s[D], s[i], n);
          int p1 = dif[t], p2 = dif[h];
          swap(s[D][p1], s[D][p2]);
          bool can = true;
          for (int l = 0; l < k && can; ++l) {
            int difs = 0;
            for (int m = 0; m < n && difs <= 2; ++m) {
              difs += s[D][m] != s[l][m];
            }
            can = can && (difs == 0 || difs == 2);
          }
          if (can) {
            puts(s[D]);
            return 0;
          }
        }
      }
      return puts("-1"), 0;
    }
  }
  if (!has_dif) {
    swap(s[0][0], s[0][1]);
    puts(s[0]);
    return 0;
  }
  puts("-1");
  return 0;
}
int main(int argc, char* argv[]) {
  solve();
  return 0;
}
