include [HTML_REMOVED]

using namespace std;
string a[201];
int main()
{
int num;
while(cin >> num ,num!=0)
{

        for(int i=0;i<num;i++)  cin >> a[i],reverse(a[i].begin(),a[i].end());
        sort(a,a+num);
        string ss="";
         for(int i=0;i<a[0].length();i++)
         {
             if(a[0][i]==a[num-1][i]) ss =a[0][i]+ss;
             else break;
         }

        cout << ss <<endl;

}

return 0;

}