#include <bits/stdc++.h>
using namespace std;
int gcd1(int a, int b) {
  if (a == 0) return b;
  return gcd1(b % a, a);
}
long long modx(long long base, long long ex) {
  long long ans = 1LL, val = base;
  while (ex > 0LL) {
    if (ex & 1LL) ans = (ans * val) % 1000000007LL;
    val = (val * val) % 1000000007LL;
    ex = ex >> 1LL;
  }
  return ans;
}
const int maxn = 1e5 + 10;
vector<int> adj[27];
bool visit[27], cycle, reckstack[27], mark[27];
int n, m;
string temp, ans, s1;
set<string> res;
stack<int> s;
void dfs(int start) {
  visit[start] = true;
  for (int i = 0; i < adj[start].size(); i++) {
    int pt = adj[start][i];
    if (!visit[pt]) {
      dfs(pt);
    }
  }
  s.push(start);
}
void dfs2(int start) {
  visit[start] = true;
  reckstack[start] = true;
  temp += (char)('a' + start);
  for (int i = 0; i < adj[start].size(); i++) {
    int pt = adj[start][i];
    if (!visit[pt]) dfs2(pt);
    if (reckstack[pt]) cycle = true;
  }
  reckstack[start] = false;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> s1;
    if (s1.length() == 1) mark[s1[0] - 'a'] = true;
    for (int j = 0; j < s1.length() - 1; j++) {
      int p = s1[j] - 'a';
      int q = s1[j + 1] - 'a';
      if (adj[p].size() > 0 && adj[p][0] == q)
        continue;
      else if (!adj[p].size())
        adj[p].push_back(q);
      else
        cycle = true;
    }
  }
  if (cycle) {
    cout << "NO" << endl;
    return 0;
  }
  for (int i = 0; i < 26; i++) {
    if (!visit[i] && (adj[i].size() || mark[i])) {
      dfs(i);
    }
  }
  if (cycle) {
    cout << "NO" << endl;
    return 0;
  }
  memset(visit, 0, sizeof(visit));
  while (!s.empty()) {
    int p = s.top();
    s.pop();
    if (!visit[p]) {
      temp = "";
      dfs2(p);
      res.insert(temp);
    }
  }
  set<string>::iterator it;
  for (it = res.begin(); it != res.end(); it++) {
    ans += *(it);
  }
  cout << ans << endl;
  return 0;
}
