#include <bits/stdc++.h>
using namespace std;
vector<set<long long>> g(26);
vector<long long> in(26, 0);
vector<long long> used(26, 0);
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  while (n--) {
    string s;
    cin >> s;
    for (long long i = 1; i < s.length(); i++) {
      if (g[s[i - 1] - 'a'].find(s[i] - 'a') == g[s[i - 1] - 'a'].end())
        in[s[i] - 'a']++;
      g[s[i - 1] - 'a'].insert(s[i] - 'a');
    }
    for (char c : s) used[c - 'a'] = 1;
  }
  queue<long long> q;
  for (long long i = 0; i < 26; i++)
    if (!in[i] && used[i])
      q.push(i);
    else if (in[i] > 1) {
      cout << "NO";
      return 0;
    }
  vector<long long> usedd(26, 0);
  string ans = "";
  while (!q.empty()) {
    int cur = q.front();
    q.pop();
    bool done = false;
    ans = ans + (char)(cur + 'a');
    while (!done) {
      usedd[cur] = 1;
      if (g[cur].size() == 1 && !usedd[*g[cur].begin()]) {
        cur = *g[cur].begin();
        ans = ans + (char)(cur + 'a');
      } else if (g[cur].size() == 0)
        done = true;
      else {
        cout << "NO";
        return 0;
      }
    }
  }
  for (long long i = 0; i < 26; i++)
    if (!usedd[i] && used[i]) {
      cout << "NO";
      return 0;
    }
  cout << (ans.length() ? ans : "NO");
  return 0;
}
