#include <bits/stdc++.h>
#include <chrono>
using namespace std;
using namespace chrono;
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define rep1(i, n) for (ll i = 1; i <= (n); ++i)
#define rrep(i, n) for (ll i = n; i > 0; --i)
#define bitrep(i, n) for (ll i = 0; i < (1 << n); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define yesNo(b) ((b) ? "Yes" : "No")
using ll = long long;
using ull = unsigned long long;
using ld = long double;
string alphabet = "abcdefghijklmnopqrstuvwxyz";
string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const double pi = 3.141592653589793;
ll smallMOD = 998244353;
ll bigMOD = 1000000007;
int dx[] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy[] = {0, 1, 0, -1, 1, 1, -1, -1};

int main()
{
    ll n;
    cin >> n;
    map<string, ll> s;
    rep(i, n)
    {
        string tmp;
        cin >> tmp;
        sort(all(tmp));
        s[tmp]++;
    }
    vector<string> v;
    for (auto x : s)
    {
        if (x.second == 1)
            v.push_back(x.first);
    }
    if (v.size() == 0)
    {
        cout << -1 << endl;
        return 0;
    }
    else
    {
        cout << v[0] + "x" << endl;
    }
    return 0;
}