#include <bits/stdc++.h>
using namespace std;
bool ok[256];
bool used[256];
char nx[256];
char pr[256];
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    char ls = 0;
    for (auto c : s) {
      ok[c] = true;
      if (ls != 0) {
        if ((nx[ls] != 0 && nx[ls] != c) || (pr[c] != 0 && pr[c] != ls)) {
          cout << "NO";
          return 0;
        }
        nx[ls] = c;
        pr[c] = ls;
      }
      ls = c;
    }
  }
  string ans = "";
  for (char c = 'a'; c <= 'z'; c++) {
    if (ok[c] && pr[c] == 0) {
      char d = c;
      while (d != 0 && !used[d]) {
        used[d] = true;
        ans.push_back(d);
        d = nx[d];
      }
      if (used[d]) {
        cout << "NO";
        return 0;
      }
    }
  }
  for (char c = 'a'; c <= 'z'; c++) {
    if (ok[c] && !used[c]) {
      cout << "NO";
      return 0;
    }
  }
  cout << ans;
  return 0;
}
