#include <bits/stdc++.h>
using namespace std;
int k, n;
bool contain_same;
string s[5005], t[5005];
bool check(int x, int y) {
  swap(s[0][x], s[0][y]);
  for (int i = 1; i < k; ++i) {
    int dif = 0;
    for (int j = 0; j < n; ++j) {
      if (s[0][j] != s[i][j]) ++dif;
    }
    if (dif == 2 || (dif == 0 && contain_same)) continue;
    swap(s[0][x], s[0][y]);
    return 0;
  }
  return 1;
}
signed main() {
  scanf("%d%d", &k, &n);
  for (int i = 0; i < k; ++i) {
    cin >> s[i];
    t[i] = s[i];
    sort(t[i].begin(), t[i].end());
  }
  for (int i = 1; i < k; ++i)
    if (t[i] != t[i - 1]) return puts("-1"), 0;
  int alp[26] = {};
  for (auto ch : s[0]) alp[ch - 'a']++;
  for (int i = 0; i < 26; ++i)
    if (alp[i] > 1) contain_same = 1;
  int ma_val = 0, ma_id = 0;
  for (int i = 1; i < k; ++i) {
    int cnt = 0;
    for (int j = 0; j < n; ++j) {
      if (s[0][j] != s[i][j]) ++cnt;
    }
    if (cnt > 4)
      return puts("-1"), 0;
    else if (ma_val < cnt) {
      ma_val = cnt;
      ma_id = i;
    }
  }
  if (ma_val == 0) {
    swap(s[0][0], s[0][1]);
    cout << s[0];
    return 0;
  }
  if (ma_val == 2) {
    if (contain_same) {
      cout << s[0];
      return 0;
    }
    return puts("-1"), 0;
  }
  vector<int> pos;
  for (int i = 0; i < n; ++i)
    if (s[0][i] != s[ma_id][i]) pos.push_back(i);
  for (auto x : pos) {
    for (int y = 0; y < n; ++y) {
      if (x == y) continue;
      if (check(x, y)) {
        cout << s[0];
        return 0;
      }
    }
  }
  puts("-1");
  return 0;
}
