#include <bits/stdc++.h>
const int MAXN = 100005;
using namespace std;
void goodbye() {
  cout << "NO\n";
  exit(EXIT_SUCCESS);
}
void check_occ(const string &s) {
  int occ[26] = {0};
  for (char c : s) {
    occ[c - 'a']++;
    if (occ[c - 'a'] > 1) {
      goodbye();
    }
  }
}
string merge(string s1, string s2) {
  int occ[26];
  fill(occ, occ + 26, -1);
  for (int i = 0; i < s1.size(); i++) {
    occ[s1[i] - 'a'] = i;
  }
  int pos1 = -1, pos2 = -1;
  for (int i = 0; i < s2.size(); i++) {
    if (occ[s2[i] - 'a'] != -1) {
      pos1 = occ[s2[i] - 'a'];
      pos2 = i;
      break;
    }
  }
  if (pos1 == -1) {
    return string();
  }
  if (pos1 < pos2) {
    swap(pos1, pos2);
    swap(s1, s2);
  }
  for (int i = 0; pos1 - i >= 0 && pos2 - i >= 0; i++) {
    if (s1[pos1 - i] != s2[pos2 - i]) {
      goodbye();
    }
  }
  for (int i = 0; pos1 + i < s1.size() && pos2 + i < s2.size(); i++) {
    if (s1[pos1 + i] != s2[pos2 + i]) {
      goodbye();
    }
  }
  string res = s1;
  if (s1.size() - pos1 >= s2.size() - pos2) {
    return s1;
  }
  for (int i = pos2 + (s1.size() - pos1); i < s2.size(); i++) {
    res.push_back(s2[i]);
  }
  check_occ(res);
  return res;
}
int N;
vector<string> vec;
void add(string s) {
  for (int i = 0; i < vec.size(); i++) {
    string m = merge(s, vec[i]);
    if (!m.empty()) {
      swap(vec[i], vec.back());
      vec.pop_back();
      add(m);
      return;
    }
  }
  vec.push_back(s);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> N;
  for (int i = 0; i < N; i++) {
    string s;
    cin >> s;
    check_occ(s);
    add(s);
  }
  sort(vec.begin(), vec.end());
  for (string s : vec) {
    cout << s;
  }
  cout << '\n';
}
