#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
istream &operator>>(istream &is, modint &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint &a) { return os << a.val(); }
istream &operator>>(istream &is, modint998244353 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint998244353 &a) { return os << a.val(); }
istream &operator>>(istream &is, modint1000000007 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint1000000007 &a) { return os << a.val(); } 

typedef long long ll;
typedef vector<vector<int>> Graph;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define FOR(i,l,r) for (int i = l;i < (int)(r); i++)
#define rep(i,n) for (int i = 0;i < (int)(n); i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define my_sort(x) sort(x.begin(), x.end())
#define my_max(x) *max_element(all(x))
#define my_min(x) *min_element(all(x))
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int INF = (1<<30) - 1;
const ll LINF = (1LL<<62) - 1;
const int MOD = 998244353;
const int MOD2 = 1e9+7;
const double PI = acos(-1);
vector<int> di = {1,0,-1,0};
vector<int> dj = {0,1,0,-1};

#ifdef LOCAL
#  include <debug_print.hpp>
#  define debug(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define debug(...) (static_cast<void>(0))
#endif

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    int N; cin >> N;
    vector<string> S(N);
    rep(i,N) cin >> S[i];

    vector<vector<int>> g(11);
    rep(i,N)g[(int)S[i].size()].push_back(i);
    
    for(int l = 1; l <= 10; l++){
        vector cnt(N, vector<int>(26));
        vector<int> mx(26);
        for(auto &idx : g[l]){
            for(int j = 0; j < (int)S[idx].size(); j++){
                cnt[idx][S[idx][j] - 'a']++;
            }
            for(int c = 0; c < 26; c++){
                mx[c] = max(mx[c], cnt[idx][c]);
            }
        }

        for(auto &i : g[l]){
            bool f = true;
            for(auto &j : g[l]){
                if(i == j) continue;
                bool issame = true;
                for(int k = 0; k < 26; k++){
                    if(cnt[i][k] != cnt[j][k]) issame = false;
                }
                if(issame) f = false;
            }
            if(f){
                for(int c = 0; c < 26; c++){
                    if(cnt[i][c] && cnt[i][c] == mx[c]){
                        cout << S[i] + char('a' + c) << endl;
                        return 0;
                    }
                }
            }
        }
    }
    cout << -1 << endl;
}