#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];
    }
    map<string, int> mp;
    for (int i = 0; i < n; i++) {
        for (char c = 'a'; c <= 'z'; c++) {
            string t = s[i] + c;
            sort(t.begin(), t.end());
            mp[t]++;
        }
    }
    for (auto [t, cnt] : mp) {
        if (cnt == 1) {
            cout << t << endl;
            return 0;
        }
    }
    cout << -1 << endl;
}