#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 20;
const int MOD = 1e9 + 7;
const int alp = 26;
int v[alp][alp], h[alp];
int main() {
  int n;
  cin >> n;
  memset(h, -1, sizeof(h));
  while (n--) {
    string s;
    cin >> s;
    for (int i = 0; i < s.length() - 1; i++) v[s[i] - 'a'][s[i + 1] - 'a'] = 1;
    for (auto ch : s) h[ch - 'a'] = 0;
  }
  string ans;
  for (int st = 0; st < alp; st++)
    if (h[st] != -1) {
      int n1 = 0;
      for (int i = 0; i < alp; i++) n1 += v[i][st];
      if (n1) continue;
      int cur = st;
      while (1) {
        h[cur] = 1;
        ans.push_back(cur + 'a');
        int deg = count(v[cur], v[cur] + alp, 1);
        if (deg > 1) {
          cout << "NO";
          return 0;
        }
        if (!deg) break;
        int ncur = 0;
        while (!v[cur][ncur]) ncur++;
        if (h[ncur]) {
          cout << "NO";
          return 0;
        }
        cur = ncur;
      }
    }
  if (count(h, h + alp, 0)) {
    cout << "NO";
    return 0;
  }
  cout << ans;
}
