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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    vector<string> a(n);
    for(auto &&s : a){
        cin >> s;
        sort(s.begin(), s.end());
    }
    sort(a.begin(), a.end());
    if(a[0] == a.back()){
        cout << "-1\n";
        return 0;
    }
    cout << 'a' << a[0] << '\n';
}