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

int main() {
  int N; cin >> N;
  vector<string> S(N); for (string &T : S) cin >> T;
  if (N == 1) {
    cout << S.at(0) + 'a' << endl;
    return 0;
  }

  for (int i = 0; i < N - 1; i++) {
    bool ok = true;
    vector<int> A(26, 0);
    for (char C : S.at(i)) A.at(C - 'a')++;
    for (int j = i + 1; j < N; j++) {
      int diff = 0;
      vector<int> B(26, 0);
      for (char C : S.at(j)) B.at(C - 'a')++;
      for (int k = 0; k < 26; k++) diff += abs(A.at(k) - B.at(k));
      if (S.at(i).size() == S.at(j).size() && diff <= 2) ok = false;
    }
    if (ok) {
      cout << S.at(i) + 'a' << endl;
      return 0;
    }
  }

  cout << -1 << endl;
}