#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1e9 + 10;
const ll INFL = 4e18;

int main() {
    int N;
    cin >> N;
    vector<string> S(N);
    for (int i = 0; i < N; i++) {
        cin >> S[i];
    }

    for (int i = 0; i < N; i++) {
        for (char c = 'a'; c <= 'z'; c++) {
            S[i].push_back(c);

            vector<int> cnt(26, 0);
            for (int j = 0; j < S[i].size(); j++) {
                cnt[S[i][j] - 'a']++;
            }

            bool flag = true;
            for (int j = 0; j < N; j++) {
                if (i == j) {
                    continue;
                }

                vector<int> cnt2(26, 0);
                for (int k = 0; k < S[j].size(); k++) {
                    cnt2[S[j][k] - 'a']++;
                }

                bool ok = false;
                for (int k = 0; k < 26; k++) {
                    if (cnt[k] < cnt2[k]) {
                        ok = true;
                        break;
                    }
                }
                if (S[j].size() + 1 != S[i].size()) {
                    ok = true;
                }

                if (!ok) {
                    flag = false;
                    break;
                }
            }

            if (flag) {
                cout << S[i] << endl;
                return 0;
            }

            S[i].pop_back();
        }
    }

    cout << -1 << endl;
}