#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool uin(T& a, T b) {
  return a > b ? (a = b, true) : false;
}
template <class T>
inline bool uax(T& a, T b) {
  return a < b ? (a = b, true) : false;
}
int edge[26][26], h[26];
string ans;
int N;
void bye() {
  cout << "NO" << '\n';
  exit(0);
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> N;
  for (int i = 0; i < N; ++i) {
    string s;
    cin >> s;
    for (int j = 0; j < (int)s.length() - 1; ++j) {
      edge[(int)s[j] - 'a'][(int)s[j + 1] - 'a'] = 1;
    }
    for (char ch : s) h[(int)ch - 'a'] = 1;
  }
  for (int alp = 0; alp < 26; ++alp) {
    if (!h[alp]) continue;
    int prev = 0;
    for (int i = 0; i < 26; ++i) prev += edge[i][alp];
    if (prev) continue;
    int cur_alp = alp;
    while (true) {
      h[cur_alp] = 2;
      ans += (char)('a' + cur_alp);
      int deg = count(edge[cur_alp], edge[cur_alp] + 26, 1);
      if (deg > 1) bye();
      if (deg == 0) break;
      int nxt = -1;
      for (int i = 0; i < 26; ++i)
        if (edge[cur_alp][i] == 1) nxt = i;
      if (h[nxt] == 2) bye();
      cur_alp = nxt;
    }
  }
  if (count(h, h + 26, 1)) bye();
  cout << ans << '\n';
}
