#ifdef LOCAL
#include <local.hpp>
#else
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2")
#include <bits/stdc++.h>
#define debug(...) ((void)0)
#define postprocess(...) ((void)0)
#endif

using namespace std;
using ll = long long;
using ld = long double;

bool is_chimatagram(string S, string T) {
    if (S.size() + 1 != T.size()) return false;

    map<char, int> m;

    for (auto&& c : S) {
        m[c]--;
    }
    for (auto&& c : T) {
        m[c]++;
    }

    for (auto&& [c, num] : m) {
        if (num != 0 && num != 1) return false;
    }

    return true;
}

void solve([[maybe_unused]] int test) {
    int N;
    cin >> N;

    vector<string> S(N);
    for (auto&& s : S) {
        cin >> s;
    }

    for (auto&& s : S) {
        for (char c = 'a'; c <= 'z'; c++) {
            string T = s + c;

            int cnt = 0;
            for (int i = 0; i < N; i++) {
                cnt += is_chimatagram(S[i], T);
            }

            if (cnt == 1) {
                cout << T << endl;
                return;
            }
        }
    }

    cout << -1 << endl;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    for (int i = 1; i <= t; i++) {
        solve(i);
    }

    postprocess();
}