#include <iostream>

#include <string>

#include <vector>

#include <algorithm>

#include <cstdio>

using namespace std;

vector<string> v;

int main(){

//    freopen("in.txt", "r", stdin);

    int n;

    while(scanf("%d", &n) && n){

        v.clear();

        string s;

        for(int i = 0; i < n; ++i){

            cin >> s;

            v.push_back(s);

        }

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

        int len = v.size();

        string s1 = v[len/2 - 1];

        string s2 = v[len/2];

        len = s1.size();

        int p = 0;

        string ans = "A";

        while(p < len){

            while(ans[p] <= 'Z' && ans < s1)  ++ans[p];

            if(ans[p] <= 'Z' && ans >= s1 && ans < s2)  break;

            if(ans[p] != s1[p]) --ans[p];

            ans += 'A';

            ++p;

        }

        cout << ans << endl;

    }

    return 0;

}