#include <bits/stdc++.h>
using namespace std;
class Solution {
  int n;
  int nxt[26], pev[26];
  bool inst[26] = {0}, vis[26] = {0}, occ[26] = {0};
  string ans, s;
  void dfs(int v) {
    if (inst[v]) {
      cout << "NO\n";
      exit(0);
    }
    ans.push_back(v + 'a');
    vis[v] = true;
    inst[v] = true;
    if (nxt[v] != -1) dfs(nxt[v]);
    inst[v] = false;
  }

 public:
  void run() {
    memset(nxt, -1, sizeof(nxt));
    memset(pev, -1, sizeof(pev));
    cin >> n;
    while (n--) {
      cin >> s;
      for (int i = 0; i < s.size() - 1; ++i) {
        if (nxt[s[i] - 'a'] != -1 and nxt[s[i] - 'a'] != s[i + 1] - 'a')
          return cout << "NO\n", void();
        nxt[s[i] - 'a'] = s[i + 1] - 'a';
        if (pev[s[i + 1] - 'a'] != -1 and pev[s[i + 1] - 'a'] != s[i] - 'a')
          return cout << "NO\n", void();
        pev[s[i + 1] - 'a'] = s[i] - 'a';
      }
      for (char c : s) occ[c - 'a'] = 1;
    }
    for (int i = 0; i < 26; ++i)
      if (pev[i] == -1 and not vis[i] and occ[i]) dfs(i);
    if (ans.size() != count(occ, occ + 26, true)) return cout << "NO\n", void();
    cout << ans << '\n';
  }
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  Solution().run();
  return 0;
}
