#include <bits/stdc++.h> 
using namespace std;

int main(){
    int N;
    cin >> N;
    vector<string>v(N);
    for (int i=0;i<N;i++){
        cin >> v.at(i);
    }

    map<string,int> d;

    for (auto s:v){
        for (char i='a';i<='z';i++){
            string t = s+i;
            sort(t.begin(),t.end());
            d[t]++;
        }
    }

    for (auto [k,v]:d){
        if (v==1){
            cout << k << endl;
            exit(0);
        }
    }

    cout << "-1" << endl;

}