#include <bits/stdc++.h>
using namespace std;
vector<bool> used;
vector<vector<int> > g, rg;
vector<int> ord;
int main() {
  int n;
  cin >> n;
  vector<string> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  g.resize(26, vector<int>(26)), rg.resize(26, vector<int>(26));
  used.resize(26);
  vector<bool> uc(26);
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < (int)a[i].size(); j++) {
      uc[a[i][j] - 'a'] = 1;
      if (j == (int)a[i].size() - 1) break;
      char c1 = a[i][j];
      char c2 = a[i][j + 1];
      g[c1 - 'a'][c2 - 'a'] = 1;
      rg[c2 - 'a'][c1 - 'a'] = 1;
    }
  }
  for (int i = 0; i < 26; i++) {
    int cnt = count(g[i].begin(), g[i].end(), 1);
    if (cnt > 1) {
      cout << "NO" << endl;
      return 0;
    }
  }
  string ans;
  int cnt = 0;
  for (int i = 0; i < 26; i++) {
    if (!uc[i]) continue;
    cnt++;
    if (count(rg[i].begin(), rg[i].end(), 1) > 0) continue;
    int u = i;
    while (1) {
      ans += (u + 'a');
      used[u] = 1;
      int pos = find(g[u].begin(), g[u].end(), 1) - g[u].begin();
      if (pos == 26) break;
      if (used[pos]) {
        cout << "NO" << endl;
        return 0;
      }
      u = pos;
    }
  }
  if (ans.size() != cnt) {
    cout << "NO" << endl;
    return 0;
  }
  cout << ans << endl;
  return 0;
}
