#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  string s;
  vector<char> path(26, '.');
  vector<char> pathr(26, '.');
  vector<bool> have(26, 0);
  for (long long i = 0; i < n; i++) {
    string t;
    cin >> t;
    if (t.size() > 26) {
      cout << "NO";
      return 0;
    }
    vector<bool> curp(26, 0);
    long long pos = -1;
    for (auto &x : t) {
      if (curp[x - 'a']) {
        cout << "NO";
        return 0;
      }
      curp[x - 'a'] = 1;
      have[x - 'a'] = 1;
    }
    for (long long i = 1; i < t.size(); i++) {
      if (path[t[i - 1] - 'a'] == '.')
        path[t[i - 1] - 'a'] = t[i];
      else if (path[t[i - 1] - 'a'] != t[i]) {
        cout << "NO";
        return 0;
      }
      if (pathr[t[i] - 'a'] == '.')
        pathr[t[i] - 'a'] = t[i - 1];
      else if (pathr[t[i] - 'a'] != t[i - 1]) {
        cout << "NO";
        return 0;
      }
    }
  }
  bool flag = 0;
  vector<bool> used(26, 0);
  for (long long i = 0; i < 26; i++) {
    if (have[i] && !used[i] && pathr[i] == '.') {
      flag = 1;
      long long cur = i;
      while (path[cur] != '.') {
        s.push_back(cur + 'a');
        used[cur] = 1;
        cur = path[cur] - 'a';
        if (used[cur]) {
          cout << "NO";
          return 0;
        }
      }
      used[cur] = 1;
      s.push_back(cur + 'a');
    }
  }
  for (long long i = 0; i < 26; i++) {
    if (!used[i] && (pathr[i] != '.' || path[i] != '.')) {
      cout << "NO";
      return 0;
    }
  }
  if (!flag) {
    cout << "NO";
    return 0;
  }
  cout << s;
  cin >> n;
  return 0;
}
