#include <iostream>
#include <vector>
#include <string>
#include <map>

using namespace std;

int main () {
    int N; cin >> N;

    vector<vector<int>> count(N, vector<int>(26));

    for (int i = 0; i < N; i++) {
        string s; cin >> s;
        for (auto& c : s) count[i][c - 'a']++;
    }

    map<vector<int>, int> mp;

    for (int i = 0; i < N; i++) {
        for (int k = 0; k < 26; k++) {
            count[i][k]++;
            mp[count[i]]++;
            count[i][k]--;
        }
    }

    for (auto v : mp) {
        if (v.second == 1) {
            for (int i = 0; i < v.first.size(); i++) {
                for (int j = 0; j < v.first[i]; j++) {
                    cout << static_cast<char>('a' + i);
                }
            }
            cout << "\n";
            return 0;
        }
    }

    cout << -1 << "\n";
}