#include <iostream>
#include <string.h>
#include <algorithm>
using namespace std; 
/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int minLen = 256;

int main(int argc, char** argv) {
  int n;
  int ans = 0;
  string s[110];
  cin >> n;
  cin.get();
  
  for(int i = 0; i < n; i++){
    getline(cin, s[i]);
    int len = s[i].length();
    if(len < minLen) minLen = len;
    reverse(s[i].begin(), s[i].end());
  }
  
  for(int i = 0; i < minLen; i++){
    char c = s[0][i];
    bool same = true;
    for(int j = 1; j < n; j++){
      if(c != s[j][i]){
        same = false;
        break;
      }
    }
    if(same == true){
      ans++;
    }else{
      break;
    }
  }
  
  if(ans){
    for(int i = ans - 1; i >= 0; i--){
      cout << s[0][i];
    }
  }else{
    cout << "nai";
  }
  
  return 0;
}