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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, j = 0;
    cin >> n;
    vector<string> a(n), S(26 * n);
    for(auto &&s : a){
        cin >> s;
        sort(s.begin(), s.end());
        for(char c = 'a'; c <= 'z'; c++){
            S[j] = s;
            S[j].insert(upper_bound(S[j].begin(), S[j].end(), c), c);
            j++;
        }
    }
    sort(S.begin(), S.end());
    for(int i = 0; i < S.size(); ){
        int p = i;
        while(i < S.size() && S[i] == S[p]) i++;
        if(i - p == 1){
            cout << S[p] << '\n';
            return 0;
        }
    }
    cout << -1 << '\n';
}