#include <bits/stdc++.h>
using namespace std;
using db = long double;
using pll = pair<long long, long long>;
std::set<long long> g[27];
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t, i, x, j, y, z, k, n;
  set<long long> let;
  long long tt;
  cin >> tt;
  for (long long _tt = 0; _tt < tt; _tt++) {
    string s;
    cin >> s;
    n = s.size();
    for (auto zx : s) {
      let.insert(zx - 'a');
    }
    for (i = 1; i < n; i += 1) {
      j = s[i - 1] - 'a';
      k = s[i] - 'a';
      g[j].insert(k);
    }
  }
  string ans = "";
  bool br = false;
  long long used[27];
  for (i = 0; i < 27; i += 1) used[i] = 0;
  for (auto zx : let) {
    if (g[zx].size() > 1) br = true;
  }
  if (!br)
    for (auto zx : let) {
      bool f = false;
      for (i = 0; i < 27; i += 1) {
        if (g[i].count(zx) > 0) f = true;
      }
      if (f || used[zx]) continue;
      i = zx;
      used[i] = 1;
      ans += char('a' + i);
      while (g[i].size()) {
        i = *g[i].begin();
        if (used[i]) {
          br = true;
          break;
        }
        used[i] = 1;
        ans += char('a' + i);
      }
    }
  if (br || ans.size() == 0) ans = "NO";
  cout << ans << '\n';
}
