#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
using mint = modint998244353;

int main() {
  int N; cin >> N;
  vector<string> s(N);
  for (int i = 0; i < N; i++) {
    cin >> s[i];
  }
  map<string, int> cnt;
  for (int i = 0; i < N; i++) {
    for (char c = 'a'; c <= 'z'; c++) {
      string t = s[i] + c;
      sort(t.begin(), t.end());
      cnt[t]++;
    }
  }
  for (auto [t, _]: cnt) {
    if (cnt[t] == 1) {
      cout << t << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}