#include <iostream>
#include <cstdio>
#include <string>
#include <algorithm>
#include <cmath>
using namespace std;

//模拟从后往前比较末尾字符串，存储后的结果翻转一下
string str[105];
int pos[105];
int main() {
    int n, i, loop = 260, j;
    cin >> n;
    getchar();
    for (i = 0; i < n; i++) {
        getline(cin, str[i], '\n');
        loop = min(loop, (int)str[i].size());
    }
    int cur_rpos = 0;
    string ans_str = "";
    for (i = 0; i < loop; i++) {
        char ch = str[0][str[0].size() - cur_rpos - 1];
        int ok = 1;
        for (j = 1; j < n; j++) {
            if (str[j][str[j].size() - cur_rpos - 1] != ch) {
                ok = 0;
                break;
            }
        }
        if (ok) ans_str += ch;
        else break;
        cur_rpos++;
    }
    if (ans_str == "") cout << "nai" << endl;
    else {
        reverse(ans_str.begin(), ans_str.end());
        cout << ans_str << endl;
    }
    return 0;
}