#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  vector<int> next(30, -1);
  vector<bool> check(26), flag(26);
  vector<int> pos;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    flag[(int)(s[0] - 'a')] = 1;
    for (int j = 0; j < s.length() - 1; j++) {
      int cur1 = (int)(s[j] - 'a'), cur2 = (int)(s[j + 1] - 'a');
      flag[cur1] = 1;
      flag[cur2] = 1;
      check[cur2] = 1;
      if (next[cur1] == -1) {
        next[cur1] = cur2;
      } else if (next[cur1] != cur2) {
        cout << "NO" << endl;
        return 0;
      }
    }
  }
  for (int i = 0; i < 26; i++) {
    vector<int> used2(26);
    int cur = i;
    used2[cur] = 1;
    while (next[cur] != -1) {
      cur = next[cur];
      if (used2[cur] == 1) {
        cout << "NO" << endl;
        return 0;
      }
      used2[cur] = 1;
    }
  }
  for (int i = 0; i < 26; i++) {
    if (!check[i] && flag[i]) {
      pos.push_back(i);
    }
  }
  set<string> s;
  vector<bool> used(26);
  string tmp = "";
  for (int i = 0; i < pos.size(); i++) {
    if (used[pos[i]]) {
      continue;
    }
    int cur = pos[i];
    used[cur] = 1;
    tmp += (char)(cur + 'a');
    while (next[cur] != -1) {
      cur = next[cur];
      if (used[cur]) {
        cout << "NO" << endl;
        return 0;
      }
      tmp += (char)(cur + 'a');
      used[cur] = 1;
    }
    s.insert(tmp);
    tmp = "";
  }
  for (auto it = s.begin(); it != s.end(); it++) {
    cout << *it;
  }
  cout << endl;
}
