PATA1077Ìâ½â

//
//  main.cpp
//  PATA1077
//
//  Created by Phoenix on 2018/2/18.
//  Copyright  2018Äê Phoenix. All rights reserved.
//

#include <iostream>
#include <cstdio>
#include <cstring>
char str[110][400];

int main(int argc, const char * argv[]) {
    int n, len = 500;
    scanf("%d", &n);
    getchar();
    for(int i = 0; i < n; i++) {
        gets(str[i]);
        if(strlen(str[i]) < len) len = strlen(str[i]);
        int j = 0, k = strlen(str[i]) - 1;
        for(j, k; j <= k; j++, k--) {
            char temp = str[i][j];
            str[i][j] = str[i][k];
            str[i][k] = temp;
        }
    }
    int k = 0;
    for(int i = 0; i < len; i++) {
        char c = str[0][i];
        bool flag = true;
        for(int j = 1; j < n; j++) {
            if(str[j][i] != c) flag = false;
        }
        if(flag == true) k++;
        else {
            break;
        }
    }
    if(k == 0) printf("nai");
    else{
        for(int i = k - 1; i >= 0; i--){
            printf("%c", str[0][i]);
        }
    }
    printf("\n");
    return 0;
}