3 4
kbub
kbbu
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 5 5
zbibx
zbbix
zbibx
xbibz
xbibz
 100 2
qz
qz
qz
qz
qz
qz
qz
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qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
zsk
kzs
kzs
kzs
skz
kzs
skz
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kzs
kzs
kzs
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kzs
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kzs
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kzs
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skz
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kzs
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kzs
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kzs
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kzs
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kzs
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kzs
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kzs
kzs
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kzs
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kzs
kzs
zsk
kzs
skz
zsk
skz
skz
skz
kzs
zsk
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
acbdh
 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>
#pragma GCC optimize("O3")
using namespace std;
inline long long getint() {
  long long _x = 0, _tmp = 1;
  char _tc = getchar();
  while ((_tc < '0' || _tc > '9') && _tc != '-') _tc = getchar();
  if (_tc == '-') _tc = getchar(), _tmp = -1;
  while (_tc >= '0' && _tc <= '9') _x *= 10, _x += (_tc - '0'), _tc = getchar();
  return _x * _tmp;
}
inline long long add(long long _x, long long _y,
                     long long _mod = 1000000007LL) {
  _x += _y;
  return _x >= _mod ? _x - _mod : _x;
}
inline long long sub(long long _x, long long _y,
                     long long _mod = 1000000007LL) {
  _x -= _y;
  return _x < 0 ? _x + _mod : _x;
}
inline long long mul(long long _x, long long _y,
                     long long _mod = 1000000007LL) {
  _x *= _y;
  return _x >= _mod ? _x % _mod : _x;
}
long long mypow(long long _a, long long _x, long long _mod) {
  if (_x == 0) return 1LL;
  long long _ret = mypow(mul(_a, _a, _mod), _x >> 1, _mod);
  if (_x & 1) _ret = mul(_ret, _a, _mod);
  return _ret;
}
long long mymul(long long _a, long long _x, long long _mod) {
  if (_x == 0) return 0LL;
  long long _ret = mymul(add(_a, _a, _mod), _x >> 1, _mod);
  if (_x & 1) _ret = add(_ret, _a, _mod);
  return _ret;
}
inline bool equal(double _x, double _y) {
  return _x > _y - 1e-9 && _x < _y + 1e-9;
}
void sleep(double sec = 1021) {
  clock_t s = clock();
  while (clock() - s < CLOCKS_PER_SEC * sec)
    ;
}
int __ = 1, _cs;
void build() {}
int n, k;
vector<string> v;
int cnt[26], tcnt[26];
bool gd;
void no() {
  puts("-1");
  exit(0);
}
void init() {
  cin >> k >> n;
  v.resize(k);
  for (int i = 0; i < k; i++) cin >> v[i];
  for (int i = 0; i < n; i++) cnt[v[0][i] - 'a']++;
  for (int i = 0; i < 26; i++)
    if (cnt[i] > 1) gd = true;
  for (int i = 1; i < k; i++) {
    for (int j = 0; j < 26; j++) tcnt[j] = 0;
    for (int j = 0; j < n; j++) tcnt[v[i][j] - 'a']++;
    for (int j = 0; j < 26; j++)
      if (cnt[j] != tcnt[j]) no();
  }
}
vector<int> dif[5050];
void gogo(int c1, int c2) {
  swap(v[0][c1], v[0][c2]);
  bool ok = true;
  for (int i = 1; i < k; i++) {
    vector<int> tdif = dif[i];
    tdif.push_back(c1);
    tdif.push_back(c2);
    sort(tdif.begin(), tdif.end());
    tdif.resize(unique(tdif.begin(), tdif.end()) - tdif.begin());
    int dif_cnt = 0;
    for (int j : tdif)
      if (v[0][j] != v[i][j]) dif_cnt++;
    if (dif_cnt == 2) continue;
    if (dif_cnt == 0 and gd) continue;
    ok = false;
    break;
  }
  if (ok) {
    cout << v[0] << endl;
    exit(0);
  }
  swap(v[0][c1], v[0][c2]);
}
void solve() {
  if (k == 1) {
    swap(v[0][0], v[0][1]);
    cout << v[0] << endl;
    exit(0);
  }
  for (int i = 1; i < k; i++) {
    for (int j = 0; j < n; j++)
      if (v[0][j] != v[i][j]) {
        dif[i].push_back(j);
        if (dif[i].size() > 4u) no();
      }
  }
  for (int i = 0; i < n; i++)
    for (int j = i + 1; j < n; j++) gogo(i, j);
  no();
}
int main() {
  build();
  while (__--) {
    init();
    solve();
  }
}
