#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  map<string, int> mp;
  rep(i, n) {
    string s;
    cin >> s;
    rep(j, 26) {
      string t = s;
      t.push_back('a' + j);
      sort(t.begin(), t.end());
      mp[t]++;
    }
  }
  for (auto&& [k, v] : mp) {
    if (v == 1) {
      cout << k << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}