#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

void solve() {
    ll n;
    cin >> n;

    map<string, ll> mp;
    vector<string> S;
    for (ll i = 0; i < n; ++i) {
        string s;
        cin >> s;
        sort(s.begin(), s.end());
        S.push_back(s);
    }

    for (ll i = 0; i < n; ++i) {
        for (char c = 'a'; c <= 'z'; ++c) {
            string tmp(S[i]);
            tmp.push_back(c);
            ++mp[tmp];
        }
    }

    for (auto m : mp) {
        if (m.second == 1) {
            cout << m.first << endl;
            return;
        }
    }

    cout << -1 << endl;

}

int main() {
    solve();
    return 0;
}