#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
int nxt[26];
int in[26];
int present[26];
int done[26];
char s[N];
int visited[26];
int get() {
  for (int i = 0; i < 26; i++) {
    if (present[i] && !visited[i] && in[i] == 0) {
      return i;
    }
  }
  return -1;
}
int valid() {
  for (int i = 0; i < 26; i++) {
    if (done[i] != present[i]) {
      return 0;
    }
  }
  return 1;
}
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < 26; i++) {
    nxt[i] = -1;
  }
  for (int i = 1; i <= n; i++) {
    scanf("%s", s);
    vector<int> tmp(26, 0);
    present[s[0] - 'a'] = 1;
    for (int j = 1; s[j]; j++) {
      int aa = s[j - 1] - 'a';
      int bb = s[j] - 'a';
      present[bb] = 1;
      tmp[bb]++;
      if (tmp[bb] > 1) {
        cout << "NO" << endl;
        return 0;
      }
      if (nxt[aa] >= 0 && nxt[aa] != bb) {
        cout << "NO" << endl;
        return 0;
      }
      in[bb] = 1;
      nxt[aa] = bb;
    }
  }
  int cc = -1;
  vector<string> ans;
  while ((cc = get()) != -1) {
    string s;
    s = 'a' + cc;
    visited[cc] = 1;
    cc = nxt[cc];
    while (cc != -1) {
      if (visited[cc]) {
        cout << "NO" << endl;
        return 0;
      }
      visited[cc] = 1;
      s += 'a' + cc;
      cc = nxt[cc];
    }
    ans.push_back(s);
  }
  for (auto e : ans) {
    for (auto c : e) {
      done[c - 'a'] = 1;
    }
  }
  if (!valid()) {
    cout << "NO" << endl;
    return 0;
  }
  sort(ans.begin(), ans.end());
  for (int i = 0; i < ans.size(); i++) {
    for (int j = 0; j < ans[i].length(); j++) {
      printf("%c", ans[i][j]);
    }
  }
  cout << endl;
}
