#include <bits/stdc++.h>
using namespace std;
int n;
map<char, vector<char> > e;
map<char, int> cl;
map<char, bool> was;
map<char, bool> bukv;
map<char, int> c1;
vector<char> d;
string ans = "";
map<pair<char, char>, bool> m;
bool dfs(char v) {
  cl[v] = 1;
  for (auto to : e[v]) {
    if (cl[to] == 0) {
      if (dfs(to)) return true;
    } else if (cl[to] == 1) {
      return true;
    }
  }
  cl[v] = 2;
  return false;
}
bool dfs2(char v) {
  was[v] = 1;
  ans += v;
  for (auto to : e[v]) {
    dfs2(to);
  }
}
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n;
  string s;
  for (int i = 0; i < n; i++) {
    cin >> s;
    d.push_back(s[0]);
    for (int i = 1; i < s.size(); i++) {
      if (!m[{s[i - 1], s[i]}]) {
        e[s[i - 1]].push_back(s[i]);
        c1[s[i]]++;
      }
      bukv[s[i]] = 1;
      m[{s[i - 1], s[i]}] = 1;
      if (e[s[i - 1]].size() > 1) {
        cout << "NO";
        return 0;
      }
      if (c1[s[i]] > 1) {
        cout << "NO";
        return 0;
      }
    }
  }
  for (char c = 'a'; c <= 'z'; c++) {
    if (!cl[c]) {
      if (dfs(c)) {
        cout << "NO";
        return 0;
      }
    }
  }
  sort(d.begin(), d.end());
  for (auto i : d) {
    if (!was[i] && !bukv[i]) dfs2(i);
  }
  cout << ans;
  return 0;
}
