#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
#define drep(i, cc, n) for (ll i = (cc); i <= (n); ++i)
#define rep(i, n) drep(i, 0, n - 1)
#define all(a) (a).begin(), (a).end()
#define pb push_back
#define fi first
#define se second
mt19937_64 rng(chrono::system_clock::now().time_since_epoch().count());
const ll MOD1000000007 = 1000000007;
const ll MOD998244353 = 998244353;
const ll MOD[3] = {999727999, 1070777777, 1000000007};
const ll LINF = 1LL << 60LL;
const int IINF = (1 << 30) - 1;


void solve(){
    int n; cin >> n;
    vector<string> s(n);
    for(int i=0; i<n; i++) cin >> s[i];

    map<string, int> mp;
    for(int i=0; i<n; i++){
        for(int j=0; j<26; j++){
            string t = s[i] + (char)('a' + j);
            sort(all(t));
            mp[t]++;
        }
    }

    for(auto [t, cnt] : mp){
        if(cnt == 1){
            cout << t << '\n';
            return;
        }
    }

    cout << -1 << '\n';
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    int T=1;
    //cin >> T;
    while(T--) solve();
}