#include <iostream>
#include <cassert>
#include <algorithm>
#include <map>

using namespace std;

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

  string lng = "";
  for (auto [s, cnt]: mp) {
    if (cnt == 1 && s.size() > lng.size()) {
      lng = s;
    }
  }

  cout << (lng.size() > 0? "a" + lng: "-1") << endl;
  return 0;
}