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

using ll = long long;
using llu = long long unsigned;

// メイン関数
int main() {
    int n;
    cin >> n;

    vector<string> s(n);
    vector<map<char, int>> data(n);
    for (int i=0; i<n; ++i) {
        cin >> s[i];
        for (int j=0; j<s[i].size(); ++j) {
            data[i][s[i][j]]++;
        }
    }

    vector<bool> isNg(n);
    for (int i=0; i<n; ++i) {
        for (int j=i+1; j<n; ++j) {
            if (s[i].size() == s[j].size()) {
                int count = 0;
                for (char c='a'; c<='z'; ++c) {
                    if (data[i][c] != data[j][c]) { count++; }
                }
                if (count <= 2) {
                    isNg[i] = true;
                    isNg[j] = true;
                }
            }
        }
    }

    string ret = "-1";
    for (int i=0; i<n; ++i) {
        if (!isNg[i]) {
            ret = s[i] + 'a';
            break;
        }
    }

    cout << ret << endl; return 0;
}