#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long MOD = 1e9 + 7;
const double PI = acos(-1.0);
string str[101000];
int take[30];
int Next[30];
int vis[30];
string res[400];
int main() {
  int n;
  cin >> n;
  if (n > 27 * 13) {
    cout << "NO" << endl;
    return 0;
  }
  for (int i = 1; i <= n; ++i) {
    cin >> str[i];
    memset(take, 0, sizeof take);
    for (int j = 0; j < str[i].length(); ++j) {
      if (!take[str[i][j] - 'a'])
        take[str[i][j] - 'a'] = 1;
      else {
        cout << "NO" << endl;
        return 0;
      }
    }
  }
  memset(vis, 0, sizeof vis);
  memset(take, 0, sizeof take);
  memset(Next, -1, sizeof(Next));
  for (int i = 1; i <= n; ++i) {
    for (int j = 0; j < str[i].length() - 1; ++j) {
      int x = str[i][j] - 'a';
      int y = str[i][j + 1] - 'a';
      if (take[x]) {
        if (Next[x] != y) {
          if (Next[x] != -1) {
            cout << "NO" << endl;
            return 0;
          } else
            Next[x] = y;
        }
      } else {
        take[x] = 1;
        Next[x] = y;
      }
    }
    for (int j = 1; j < str[i].length(); ++j) vis[str[i][j] - 'a']++;
    take[str[i][str[i].length() - 1] - 'a'] = 1;
  }
  int p = 0;
  for (int i = 0; i <= 25; ++i) {
    if (vis[i] == 0 && take[i] == 1) {
      res[++p].push_back(i + 'a');
      int now = Next[i];
      int cnt = 0;
      while (now != -1 && cnt < 50)
        res[p].push_back(now + 'a'), cnt++, now = Next[now];
      if (cnt >= 30) {
        cout << "NO" << endl;
        return 0;
      }
    }
  }
  int cc = 0;
  for (int i = 0; i <= 25; ++i)
    if (take[i]) cc++;
  sort(res + 1, res + p + 1);
  string ans;
  ans.clear();
  for (int i = 1; i <= p; ++i) ans += res[i];
  memset(take, 0, sizeof take);
  for (int i = 0; i < ans.length(); ++i) {
    if (!take[ans[i] - 'a'])
      take[ans[i] - 'a'] = 1;
    else {
      cout << "NO" << endl;
      return 0;
    }
  }
  if (cc != ans.length()) {
    cout << "NO" << endl;
    return 0;
  }
  cout << ans << endl;
}
