#include <bits/stdc++.h>
#include <type_traits>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using ll = long long;
int n;
vector<string> S;

void solve(){
    map<string, int> mp;
    for(int i = 0;i<n;i++){
        for(int j = 0;j<26;j++){
            string t = S[i];
            t.push_back('a'+j);
            sort(t.begin(), t.end());
            mp[t]++;
        }
    }
    for(auto &i:mp){
        if(i.second==1){
            cout<<i.first<<endl;
            return;
        }
    }
    cout<<-1<<endl;
}

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n;
    S = vector<string>(n);
    for(auto &i: S)
            cin >> i;
    solve();
}