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

int main() {
    int n;
    cin >> n;
    vector<string> s(n);
    for (int i = 0; i < n; i++) {
        cin >> s[i];
    }
    vector<string> t = s;
    for (int i = 0; i < n; i++) {
        sort(t[i].begin(), t[i].end());
    }
    sort(t.begin(), t.end());
    t.erase(unique(t.begin(), t.end()), t.end());
    if (t.size() == 1) {
        cout << -1 << endl;
        return 0;
    }
    vector<vector<int>> cnt(n, vector<int>(26));
    for (int i = 0; i < n; i++) {
        for (char c : s[i]) {
            cnt[i][c - 'a']++;
        }
    }
    for (int i = 0; i < n; i++) {
        for (char c = 'a'; c <= 'z'; c++) {
            cnt[i][c - 'a']++;
            bool ok = true;
            for (int ni = 0; ni < n; ni++) {
                if (i == ni) {
                    continue;
                }
                int zero = 0, one = 0;
                for (int j = 0; j < 26; j++) {
                    if (cnt[i][j] == cnt[ni][j]) {
                        zero++;
                    }
                    if (cnt[i][j] - cnt[ni][j] == 1) {
                        one++;
                    }
                }
                if (zero == 25 && one == 1) {
                    ok = false;
                }
            }
            if (ok) {
                cout << s[i] + c << endl;
                return 0;
            }
            cnt[i][c - 'a']--;
        }
    }
}