#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) % 1000000009LL;
    val = (val * val) % 1000000009LL;
    ex = ex >> 1LL;
  }
  return ans;
}
int n;
vector<string> v;
set<int> adj[27];
set<int> adj2[27];
bool visit[27];
bool recstack[27];
bool mark[27];
bool cycle;
stack<int> s;
string str;
string ans = "";
void dfs2(int start) {
  ans += (char)(start + 'a');
  visit[start] = true;
  if (!adj[start].size()) return;
  dfs2(*adj[start].begin());
}
void dfs(int start) {
  if (cycle) return;
  visit[start] = true;
  recstack[start] = true;
  if (!adj[start].size()) {
    recstack[start] = false;
    s.push(start);
    return;
  }
  int pt = *adj[start].begin();
  if (recstack[pt]) {
    cycle = true;
    return;
  }
  if (!visit[pt]) dfs(pt);
  recstack[start] = false;
  s.push(start);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> str;
    v.push_back(str);
  }
  for (int i = 0; i < n; i++) {
    str = v[i];
    int l = str.length();
    for (int j = 0; j < l - 1; j++) {
      adj[str[j] - 'a'].insert(str[j + 1] - 'a');
      adj2[str[j + 1] - 'a'].insert(str[j] - 'a');
      if (adj[str[j] - 'a'].size() > 1 || adj2[str[j + 1] - 'a'].size() > 1) {
        cout << "NO" << endl;
        return 0;
      }
      mark[str[j] - 'a'] = true;
    }
    mark[str[l - 1] - 'a'] = true;
  }
  sort(v.rbegin(), v.rend());
  for (int i = 25; i >= 0; i--) {
    if (mark[i] && !visit[i]) dfs(i);
    if (cycle) {
      cout << "NO" << endl;
      return 0;
    }
  }
  memset(visit, false, sizeof(visit));
  while (!s.empty()) {
    int t = s.top();
    s.pop();
    if (visit[t]) continue;
    dfs2(t);
  }
  cout << ans << endl;
  return 0;
}
