#include <bits/stdc++.h>
using namespace std;
int n;
string s;
int nx[26], bk[26], app[26];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  fill(nx, nx + 26, -1);
  fill(bk, bk + 26, -1);
  cin >> n;
  while (n--) {
    cin >> s;
    for (int i = 0; i < s.size() - 1; i++) {
      if (nx[s[i] - 'a'] != -1 && !(nx[s[i] - 'a'] == s[i + 1] - 'a')) {
        cout << "NO";
        return 0;
      } else {
        nx[s[i] - 'a'] = s[i + 1] - 'a';
        bk[s[i + 1] - 'a'] = s[i] - 'a';
      }
    }
    if (s.size() == 1) app[s[0] - 'a'] = 1;
  }
  vector<string> res;
  bool flag = false;
  for (int i = 0; i < 26; i++) {
    string x;
    if (bk[i] == -1 && nx[i] != -1) {
      flag = true;
      int c = i;
      do {
        x.push_back(c + 'a');
        c = nx[c];
      } while (nx[c] != -1);
      x.push_back(c + 'a');
    } else if (app[i]) {
      flag = true;
      x.push_back(i + 'a');
    }
    res.push_back(x);
  }
  if (!flag) {
    cout << "NO";
    return 0;
  }
  sort(res.begin(), res.end());
  for (string x : res) cout << x;
}
