#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 998244353;
// constexpr int MOD = 1000000007;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n; cin >> n;
  vector<string> s(n);
  for (string& s_i : s) {
    cin >> s_i;
    ranges::sort(s_i);
  }
  REP(i, n) {
    bool is_valid = true;
    REP(j, n) {
      if (j == i) continue;
      for (int k = 0; k + s[i].length() <= s[j].length(); ++k) {
        if (s[i] == s[j].substr(k, s[i].length())) {
          is_valid = false;
          break;
        }
      }
      if (is_valid) {
        cout << s[i] << '\n';
        return 0;
      }
    }
  }
  cout << "-1\n";
  return 0;
}