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

int main() {
  int N; cin >> N;
  string a = "abcdefghijklmnopqrstuvwxyz";
  vector<string> S(N);
  map<string, int> M;

  for (int i = 0; i < N; i++) {
    cin >> S.at(i);
    sort(S.at(i).begin(), S.at(i).end());
    M[S.at(i)]++;
  }

  for (pair<string, int> P : M) {
    if (P.second == 1) {
      for (int i = 0; i < 26; i++) {
        int Count = 0;
        string T = P.first + a.at(i);
        sort(T.begin(), T.end());
        for (int j = 0; j < N; j++) {
          for (int k = 0; k < 26; k++) {
            string U = S.at(j) + a.at(k);
            sort(U.begin(), U.end());
            if (T == U) Count++;
          }
        }
        if (Count == 1) {
          cout << T << endl;
          return 0;
        }
      }
    }
  }

  cout << -1 << endl;
}