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

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    
    int N; cin >> N;
    map<string,int> S;
    for(int i=0; i<N; i++){
        string s; cin >> s;
        sort(s.begin(),s.end());
        S[s]++;
    }

    map<string,int> S2;
    for(auto [k,v] : S){
        if(v != 1) continue;
        for(char c='a'; c<='z'; c++){
            string s = k;
            s += c;
            S2[s]++;
        }
    }
    for(auto [k,v] : S2){
        if(v == 1){cout << k << endl; return 0;}
    }
    cout << -1 << endl;
}