pat-1077.cpp

#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <unordered_map>
#include <limits>
#include <map>
#include <iterator>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>

int main() {
    std::vector<std::string> vec_str;
    int cases;

    std::cin >> cases;
    char buf[257];
    // ignore all \n
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    int min_len = std::numeric_limits<int>::max();
    for (int i = 0; i < cases; ++i) {
        std::string tmp;

        int len = tmp.length();
        min_len = std::min(len, min_len);
        vec_str.push_back(std::move(tmp));
    }
    std::string ans;
    for (int i = 0; i < min_len; ++i) {
        char ch = 0;
        for (int j = 0; j < cases; ++j) {
            int idx = vec_str[j].length() -1 - i;
            char cur = vec_str[j][idx];
            if (ch == 0) {
                ch = cur;
            } else {
                if (ch != cur) {
                    goto out;
                }
            }
        }
        ans += ch;
    }
out:
    std::reverse(std::begin(ans), std::end(ans));
    if (!ans.length()) {
        std::cout << "nai" << std::endl;
    } else {
        std::cout << ans << std::endl;
    }
    return 0;
}