#include <bits/stdc++.h>
using namespace std;
template <typename T>
void pr(vector<T> &v) {
  for (int i = 0; i < (int)(v).size(); i++) cout << v[i] << " ";
  cout << '\n';
  ;
}
template <typename T>
void pr(vector<vector<T>> &v) {
  for (int i = 0; i < (int)(v).size(); i++) {
    pr(v[i]);
  }
}
template <typename T>
void re(T &x) {
  cin >> x;
}
template <typename T>
void re(vector<T> &a) {
  for (int i = 0; i < (int)(a).size(); i++) re(a[i]);
}
template <class Arg, class... Args>
void re(Arg &first, Args &...rest) {
  re(first);
  re(rest...);
}
template <typename T>
void pr(T x) {
  cout << x << '\n';
  ;
}
template <class Arg, class... Args>
void pr(const Arg &first, const Args &...rest) {
  cout << first << " ";
  pr(rest...);
  cout << '\n';
  ;
}
void ps() {
  cout << '\n';
  ;
}
template <class T, class... Ts>
void ps(const T &t, const Ts &...ts) {
  cout << t;
  if (sizeof...(ts)) cout << " ";
  ps(ts...);
}
const long long MOD = 998244353;
long double PI = 4 * atan(1);
long double eps = 1e-12;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  int inv[30] = {};
  int ouv[30] = {};
  int vis[30] = {};
  for (int i = 0; i < n; i++) {
    string str;
    cin >> str;
    for (int j = 0; j < str.length() - 1; j++) {
      int c1 = str[j] - 'a' + 1;
      int c2 = str[j + 1] - 'a' + 1;
      if (inv[c2] != 0 && inv[c2] != c1) {
        cout << "NO" << '\n';
        ;
        return 0;
      }
      if (ouv[c1] != 0 && ouv[c1] != c2) {
        cout << "NO" << '\n';
        ;
        return 0;
      }
      inv[c2] = c1;
      ouv[c1] = c2;
      vis[c1] = 1;
      vis[c2] = 1;
    }
  }
  int vis2[30] = {};
  string ans;
  for (int i = 0; i < 30; i++) {
    int curr = i;
    if (vis[curr] == 1 && inv[curr] == 0) {
      ans += 'a' + curr - 1;
      vis2[curr] = 1;
      while (ouv[curr] != 0) {
        curr = ouv[curr];
        ans += 'a' + curr - 1;
        if (vis2[curr] == 1) {
          cout << "NO" << '\n';
          ;
          return 0;
        }
        vis2[curr] = 1;
      }
    }
  }
  cout << ans << '\n';
  ;
}
