#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for (int i = 0; i < (int)n; i++)

int main() {
  int N;
  cin >> N;
  vector<map<char, int>> S(N);
  rep(i, N) {
    string Si;
    cin >> Si;
    for (char c : Si)
      S[i][c]++;
  }
  map<map<char, int>, int> T;
  rep(i, N) {
    map<char, int> tmp = S[i];
    rep(j, 26) {
      tmp[int('a') + j]++;
      T[tmp]++;
      tmp[int('a') + j]--;
    }
  }
  auto prt = [&](map<char, int> mp) {
    for (auto it = mp.begin(); it != mp.end(); it++)
      rep(i, it->second)
	cout << it->first;
    return;
  };
  for (auto it = T.begin(); it != T.end(); it++) {
    if (it->second == 1) {
      prt(it->first); cout << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}