#include <bits/stdc++.h>
#include <type_traits>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using ll = long long;
int n;
vector<string> S;

void solve(){
    for(int i = 0;i<n;i++){
        for(int j = 0;j<26;j++){
            string t = S[i];
            t.push_back('a'+j);
            sort(t.begin(), t.end());
            bool is = true;
            for(int k = 0;k<n;k++){
                if(k==i)
                    continue;
                for(int l = 0;l<26;l++){
                    string u = S[k];
                    u.push_back('a'+l);
                    sort(u.begin(), u.end());
                    if(t == u){
                        is = false;
                        break;
                    }
                }
            }
            if(is){
                cout << t << endl;
                return;
        }
        }
    }
    cout<<-1<<endl;
}

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n;
    S = vector<string>(n);
    for(auto &i: S)
            cin >> i;
    solve();
}