#define _USE_MATH_DEFINES
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <math.h>
#include <cmath>
#include <stack>
#include <map>
#include <set>
#include <numeric>
#include <iomanip>
#include <climits>
#include <functional>
#include <cassert>
#include <tuple>
using namespace std;
using ll = long long;

int N;

int main() {

    cin >> N;
    map<string, int> mp;
    for(int i = 0; i < N; ++i) {
        string s; cin >> s;
        for(char c = 'a'; c <= 'z'; ++c) {
            string t = s;
            t.push_back(c);
            sort(t.begin(),t.end());
            mp[t]++;
        }
    }

    for(auto [s,v] : mp) {
        if(v == 1) {
            cout << s << endl;
            return 0;
        }
    }

    cout << -1 << endl;

}