#include <iostream>
#include <algorithm>
#include <map>
using namespace std;
using ll = long long;

int N;
map<string, int> cnt;
int main(void){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    
    cin >> N;
    for(int i = 0;i < N;i++){
        string s; cin >> s;
        for(char c = 'a';c <= 'z';c++){
            string t = s + c;
            sort(t.begin(), t.end());
            cnt[t]++;
        }
    }
    for(auto [s, p] : cnt){
        if(p == 1){
            cout << s << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}