#include <bits/stdc++.h>
using namespace std;
const int N = 100000;
const int inf = (int)1e9 + 1;
const long long big = (long long)1e18 + 1;
const int P = 239;
const int MOD = (int)1e9 + 7;
const int MOD1 = (int)1e9 + 9;
const double eps = 1e-9;
const double pi = atan2(0, -1);
const int ABC = 26;
string a[N];
int cnt[N][26];
bool used[26];
set<int> g[26], gr[26];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.precision(20);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    for (int j = 0; j < (int)a[i].size(); j++) {
      cnt[i][a[i][j] - 'a']++;
      used[a[i][j] - 'a'] = true;
      if (cnt[i][a[i][j] - 'a'] == 2) {
        cout << "NO\n";
        return 0;
      }
    }
  }
  for (int i = 0; i < n; i++)
    for (int j = 0; j < (int)a[i].size() - 1; j++) {
      g[a[i][j] - 'a'].insert(a[i][j + 1] - 'a');
      gr[a[i][j + 1] - 'a'].insert(a[i][j] - 'a');
    }
  for (int i = 0; i < 26; i++)
    if ((int)g[i].size() > 1) {
      cout << "NO\n";
      return 0;
    }
  vector<string> res;
  for (int i = 0; i < 26; i++)
    if (used[i] && (int)gr[i].size() == 0) {
      string t;
      int cur = i;
      while (true) {
        used[cur] = false;
        t += (char)('a' + cur);
        if ((int)g[cur].size() == 0)
          break;
        else {
          if (used[*g[cur].begin()])
            cur = *g[cur].begin();
          else {
            cout << "NO\n";
            return 0;
          }
        }
      }
      res.push_back(t);
    }
  for (int i = 0; i < 26; i++)
    if (used[i]) {
      cout << "NO\n";
      return 0;
    }
  sort(res.begin(), res.end());
  for (auto it : res) cout << it;
  cout << "\n";
  return 0;
}
