#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <string>
#include <list>
#include <cassert>
#include <numeric>
#include <cstdint>
#include <queue>
#include <deque>
#include <stack>
#include <set>
#include <random>
#include <bitset>
#include <climits>
#include <chrono>
#include <atcoder/all>
 
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using namespace std;
using namespace atcoder;
using P = pair<ll, ll>;
using Graph = vector<vector<ll>>;
using Priority = priority_queue<ll, vector<ll>, greater<ll>>;// 昇順
using PriorityPair = priority_queue<P, vector<P>, greater<P>>;
using mint17 = modint1000000007;
using mint998 = modint998244353;
using mint = modint;
 
#define mod 1000000007
#define MAX_WIDTH 60
#define MAX_HEIGHT 60
#define inf 1e9
#define INF 8e18
#define MOD 998244353
#define PI 3.141592653589793
#define rep(i, a, b) for(ll i=(a);i<(b);i++)
#define rrep(i, a, b) for(ll i=(a);i>=(b);i--)
#define fore(i, a) for(auto &i: a)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define YES(a) cout << ((a) ? "YES" : "NO") << endl
#define Yes(a) cout << ((a) ? "Yes" : "No") << endl
#define First(a) cout << ((a) ? "First" : "Second") << endl
#define pb push_back
#define fi first
#define se second
#define vi vector<int>
#define vll vector<ll>
#define vld vector<ld>
#define vs vector<string>
#define vc vector<char>
#define vp vector<pair<ll, ll>>
#define mll map<ll, ll>
#define msl map<string, ll>
#define COUT(n) cout << (n) << endl
#define isInGrid(a, b, h, w) (0 <= (a) && (a) < (h) && 0 <= (b) && (b) < (w))
#define isInRange(n, l, r) ((l) <= (n) && (n) < (r))
#define countBit(n) __builtin_popcountll(n)
template<typename T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<typename T> inline bool chmax(T& a, T b) { if (a <= b) { a = b; return true; } return false; }
template<typename T> void uniq(T &vec){sort(all(vec)); vec.erase(unique(all(vec)), vec.end());}
template <class T> bool isOverflowAdd(T a, T b) { return (numeric_limits<T>::max() - a) < b; }
template <class T> bool isOverflowMul(T a, T b) { return (numeric_limits<T>::max() / a) < b; }
int vy4[] = {0, 1, 0, -1}, vx4[] = {1, 0, -1, 0};
int vy8[] = {0, 1, 1, 1, 0, -1, -1, -1}, vx8[] = {1, 1, 0, -1, -1, -1, 0, 1};
struct edge {
    ll to, cost;
};

ll N;
string S[1010];
int main() {
    cin >> N;
    rep(i,0,N) {
        cin >> S[i];
    }

    map<string, ll> cnt;
    rep(i,0,N) {
        rep(j,0,26) {
            string t = S[i];
            t += (char)('a'+j);
            sort(all(t));
            cnt[t]++;
        }
    }

    for(auto [p,v]: cnt) {
        if(v == 1) {
            COUT(p);
            return 0;
        }
    }
    COUT(-1);

    return 0;
}