#include <bits/stdc++.h>
using namespace std;
const int N = 30 + 3;
int n, deg[N];
bool exist[N], match[N][N], vis[N], foo = true;
vector<int> adj[N];
vector<string> V;
void init() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
void dfs(int u, string s) {
  vis[u] = 1;
  s += (char)(u + 'a');
  cerr << u << ' ' << s << ' ' << adj[u].size() << '\n';
  if (adj[u].size() == 0) {
    V.push_back(s);
    return;
  }
  for (int v : adj[u])
    if (!vis[v])
      dfs(v, s);
    else
      foo = false;
}
void solve() {
  cin >> n;
  for (int i = 1; i <= (int)n; ++i) {
    string s;
    cin >> s;
    for (int j = 0; j <= (int)s.size() - 1; ++j) {
      exist[s[j] - 'a'] = 1;
      if (j < s.size() - 1) match[s[j] - 'a'][s[j + 1] - 'a'] = 1;
    }
  }
  for (int i = 0; i <= (int)25; ++i)
    for (int j = 0; j <= (int)25; ++j)
      if (match[i][j]) adj[i].push_back(j), deg[j]++;
  for (int i = 0; i <= (int)25; ++i)
    if (adj[i].size() > 1) foo = false;
  for (int i = 0; i <= (int)25; ++i)
    for (int v : adj[i])
      for (int k : adj[v])
        if (i == k) foo = false;
  for (int i = 0; i <= (int)25; ++i)
    if (deg[i] == 0 && exist[i]) dfs(i, "");
  for (int i = 0; i <= (int)25; ++i)
    if (exist[i] && !vis[i]) foo = false;
  if (!foo)
    cout << "NO";
  else
    for (auto u : V) cout << u;
}
int main() {
  init();
  solve();
}
