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

int main() {
  int N; cin >> N;
  map<string, int> M;

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

  for (pair<string, int> P : M) {
    if (P.second == 1) {
      cout << P.first + 'a' << endl;
      return 0;
    }
  }

  cout << -1 << endl;
}