#include <iostream>

 #include <cstdio>

 #include <cstring>

 #include <algorithm>

 #include <string>

 #include <vector>

 #include <map>

 #include <set>

 #define FRER() freopen("in.txt", "r", stdin)

 #define FREW() freopen("out.txt", "w", stdout)

 #define INF 0x3f3f3f3f

 using namespace std;

 int main()

 {

     //FRER();

     //FREW();

     vector<string> vec;

     string str, ptr;

     int n;

     while(cin >> n && n) {

         vec.clear();

         for(int i = ; i < n; ++i) {

             cin >> str;

             vec.push_back(str);

         }

         sort(vec.begin(), vec.end());

         int b = n / , a = b - ;

         str = vec[a];

         ptr = vec[b];

         int len = min(str.length(), ptr.length());

         bool ok = true;

         string s = "", tmp;

         for(int i = ; i < len; ++i) {

             if(str[i] == ptr[i])

                 s += str[i];

             else {

                 tmp = s + (char)(str[i] + );

                 if(tmp == ptr) {

                     s += str[i++];

                     while(i < str.length() -  && str[i] == 'Z')

                         s += str[i++];

                     if(i < str.length() - )

                         s += (char)(str[i] + );

                     else

                         s = str;

                 }

                 else {

                     if(tmp.length() == str.length())

                         s = str;

                     else

                         s = tmp;

                 }

                 break;

             }

         }

         cout << s << endl;

     }

     return ;

 }