#include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
#include <math.h>
#include <cmath> 
#include <climits>
#include <functional>
#include <iomanip>
#include <limits>
#include <cassert>
#include <random>
#include <complex>
#include <numeric>
#include <regex>
#include <list>
#include <ctime> 

using namespace std;

#define ll long long
#define ull unsigned long long
#define ld  long double
typedef vector<long long> vll;

using LL = long long; using ULL = unsigned long long;
using VI = vector<int>; using VVI = vector<VI>; using VVVI = vector<VVI>;
using VL = vector<LL>; using VVL = vector<VL>; using VVVL = vector<VVL>;
using VB = vector<bool>; using VVB = vector<VB>; using VVVB = vector<VVB>;
using VD = vector<double>; using VVD = vector<VD>; using VVVD = vector<VVD>;
using VC = vector<char>; using VS = vector<string>; using VVC = vector<VC>;
using PII = pair<int,int>; using PLL = pair<LL,LL>; using PDD = pair<double,double>; using PIL = pair<int,LL>;
using MII = map<int,int>; using MLL = map<LL,LL>;
using SI = set<int>; using SL = set<LL>;
using MSI = multiset<int>; using MSL = multiset<LL>;
template<class T> using MAXPQ = priority_queue<T>;
template<class T> using MINPQ = priority_queue< T, vector<T>, greater<T> >;

const ll MOD = 1000000007;
const ll MOD2 = 998244353;
const ll INF = 1LL << 60;
#define PI  3.14159265358979323846

#define FOR(i, a, b) for(int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define EACH(e, v) for(auto &e : v)
#define RITR(it, v) for(auto it = (v).rbegin(); it != (v).rend(); ++it)
#define ALL(v) v.begin(),v.end()

vector<ll> x8={1,1,1,0,0,-1,-1,-1},y8={1,0,-1,1,-1,1,0,-1};
int dx4[4]={1,-1,0,0}, dy4[4]={0,0,1,-1};

/*
memo
-uf,RMQ(segtree),BIT,BIT2,SegTree,SegTreeLazy
-isprime,Eratosthenes,gcdlcm,factorize,divisors,modpow,moddiv(modpow必要)
nCr(+modnCr,inverse,extend_euclid.powmod),tobaseB,tobase10
-dijkstra,Floyd,bellmanford,sccd,topological,treediamiter
-compress1,compress2,rotate90

-co,ci,fo1,fo2,fo3,fo4
-bitsearch,binaryserach
-bfs(vis.assign忘れるな)
-SegTreedec,SegTreeLazydec
*/


int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(0);
    ll N; cin >> N;
    VS s(N);
    for(ll i = 0; i < N; i++) cin >> s[i];
    VVL cnt(N,VL(26,0));
    for(ll i = 0; i < N; i++){
        sort(ALL(s[i]));
        for(ll j = 0; j < s[i].size(); j++) cnt[i][s[i][j]-'a']++;
    }

    map<VL,ll> m;
    map<string,ll> mp;
    for(ll i = 0; i < N; i++){
        for(ll j = 0; j < 26; j++){
            string tmp = s[i];
            tmp += 'a'+j;
            sort(ALL(tmp));
            mp[tmp]++;
        }
    }

    for(auto p : mp){
        if(p.second == 1){
            cout << p.first << endl;
            return 0;
        }
    }

    
    cout << -1 << endl;
}