#include<bits/stdc++.h>
using namespace std;
int N;
map<string,int>cnt;
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin>>N;
    for(;N--;)
    {
        string S;
        cin>>S;
        for(char c='a';c<='z';c++)
        {
            string T=S;
            T.push_back(c);
            sort(T.begin(),T.end());
            cnt[T]++;
        }
    }
    for(auto[key,val]:cnt)if(val==1)
    {
        cout<<key<<endl;
        return 0;
    }
    cout<<"-1"<<endl;
}