#include <bits/stdc++.h>
using namespace std;
void GO() {
  ios_base::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
}
const long long INF = 1e9 + 7;
const long long mod = (1e9) + 7;
const int maxn = (int)2e5 + 1;
const long double PI = acos(-1.0);
const long double eps = 0.000000001;
mt19937 rnd(time(0));
int g[27], u[27], us[27], p[27];
vector<int> top_sort;
void dfs(int v) {
  if (v == -1 || u[v]) return;
  u[v] = 1;
  dfs(g[v]), top_sort.push_back(v);
}
void solve() {
  int n;
  cin >> n;
  memset(g, -1, sizeof(g));
  memset(p, -1, sizeof(p));
  memset(u, 0, sizeof(u));
  memset(us, 0, sizeof(us));
  for (int i = 0; i < n; ++i) {
    string s;
    cin >> s;
    for (int j = 0; j < int(s.size()) - 1; ++j) {
      int v = s[j] - 'a';
      int to = s[j + 1] - 'a';
      us[v] = 1;
      if (g[v] == -1) {
        g[v] = to;
        if (p[to] == -1) p[to] = v;
      }
      if (g[v] != to || p[to] != v) {
        cout << "NO\n";
        return;
      }
    }
  }
  vector<string> ans;
  for (int i = 0; i < 27; ++i)
    if (us[i]) dfs(i);
  memset(u, 0, sizeof(u));
  for (int i = int(top_sort.size()) - 1; i >= 0; --i) {
    int v = top_sort[i];
    if (u[v]) continue;
    string plus;
    while (v != -1) {
      plus += char(v + 'a');
      u[v] = 1, v = g[v];
    }
    ans.push_back(plus);
  }
  sort(ans.begin(), ans.end());
  for (auto &i : ans) cout << i;
}
signed main() {
  GO();
  int Q = 1;
  while (Q--) {
    solve();
  }
}
