#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

void solve() {
  ll n;
  cin >> n;
  map<string, ll> mp;
  rep(i, n) {
    string s;
    cin >> s;
    s += 'a';
    sort(s.begin(), s.end());
    ll j = 0, sz = s.size();
    rep(_, 26) {
      while (j < sz - 1 && s[j] == s[j + 1])
        j++;
      mp[s]++;
      s[j]++;
    }
  }
  for (const auto &[k, v] : mp)
    if (v == 1) {
      cout << k << '\n';
      return;
    }
  cout << -1 << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}