#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<map<char, int>> vec(N);
  vector<bool> flag(N, true);
  for (int i = 0; i < N; i++) {
    string s; cin >> s;
    for (int j = 0; j < s.size(); j++) {
      vec[i][s[j]]++;
    }
    for (int j = 0; j < i; j++) {
      if (vec[i] == vec[j]) flag[i] = flag[j] = false;  
    }
  }
  for (int i = 0; i < N; i++) {
    if (flag[i]) {
      for (auto p: vec[i]) {
        for (int j = 0; j < p.second; j++) cout << p.first;
      }
      cout << 'a' << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}