#include <bits/stdc++.h>
using namespace std;
int a[26][2];
string s;
bool s_merge(string fir, string sec) {
  int pos1, pos2;
  for (int i = 0; i < 26; i++)
    if (a[i][0] != -1 && a[i][1] != -1) {
      pos1 = a[i][0];
      pos2 = a[i][1];
      break;
    }
  if (pos1 < pos2) {
    swap(pos1, pos2);
    swap(fir, sec);
  }
  s = fir;
  int pos;
  for (int i = 0; i < sec.length(); i++) {
    pos = i + pos1 - pos2;
    if (pos < fir.length()) {
      if (s[pos] != sec[i]) return false;
    } else
      s += sec[i];
  }
  return true;
}
bool same_sim() {
  for (int i = 0; i < 26; i++)
    if (a[i][0] != -1 && a[i][1] != -1) return true;
  return false;
}
void make_arr(string s, int arr) {
  for (int i = 0; i < 26; i++) a[i][arr] = -1;
  int num;
  for (int i = 0; i < s.length(); i++) {
    num = (int)(s[i] - 'a');
    a[num][arr] = i;
  }
}
bool check(string s) {
  bool sim[26] = {};
  int num;
  for (int i = 0; i < s.length(); i++) {
    num = (int)(s[i] - 'a');
    if (sim[num])
      return false;
    else
      sim[num] = true;
  }
  return true;
}
vector<string> st;
int main() {
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> s;
    if (check(s)) {
      bool fl = true;
      while (fl) {
        fl = false;
        make_arr(s, 1);
        for (int to = 0; to < st.size(); to++) {
          make_arr(st[to], 0);
          if (!same_sim()) continue;
          if (s_merge(st[to], s)) {
            st.erase(st.begin() + to);
            fl = true;
            break;
          } else {
            cout << "NO";
            return 0;
          }
        }
      }
      st.push_back(s);
    } else {
      cout << "NO";
      return 0;
    }
  }
  sort(st.begin(), st.end());
  for (int i = 0; i < st.size(); i++) cout << st[i];
  return 0;
}
