#include <bits/stdc++.h>
using namespace std;
char string_in_buffer[(int)2e5];
void fast_scan(int &first) { scanf("%d", &first); }
void fast_scan(long long &first) { scanf("%lld", &first); }
void fast_scan(unsigned long long &first) { scanf("%llu", &first); }
void fast_scan(double &first) { scanf("%lf", &first); }
void fast_scan(long double &first) { scanf("%Lf", &first); }
void fast_scan(char &first) {
  scanf("%c", &first);
  if (first == '\n') {
    fast_scan(first);
  }
}
void fast_scan(string &first) {
  scanf("%s", string_in_buffer);
  first = string(string_in_buffer);
}
template <class TFirst, class TSecond>
void fast_scan(pair<TFirst, TSecond> &p) {
  fast_scan(p.first);
  fast_scan(p.second);
}
template <class T>
void fast_scan(vector<T> &v) {
  for (auto &first : v) fast_scan(first);
}
void fast_print(const int &first) { printf("%d", first); }
void fast_print(const long long &first) { printf("%lld", first); }
void fast_print(const unsigned long long &first) { printf("%llu", first); }
void fast_print(const double &first) { printf("%.15lf", first); }
void fast_print(const long double &first) { printf("%.15Lf", first); }
void fast_print(const char &first) { printf("%c", first); };
void fast_print(const string &first) { printf("%s", first.c_str()); }
template <class TFirst, class TSecond>
void fast_print(const pair<TFirst, TSecond> &p) {
  fast_print(p.first);
  fast_print(' ');
  fast_print(p.second);
}
template <class T>
void fast_print(const vector<T> &v) {
  if (v.empty()) return;
  fast_print(v[0]);
  for (int i = 1; i < v.size(); i++) {
    fast_print(" ");
    fast_print(v[i]);
  }
}
template <class T>
void fast_print(const vector<vector<T>> &v) {
  if (v.empty()) return;
  fast_print(v[0]);
  for (int i = 1; i < v.size(); i++) {
    fast_print("\n");
    fast_print(v[i]);
  }
}
using namespace std;
namespace smart_io {
string print_start = "";
string sep = " ";
bool first_print = false;
void precall_print() {
  fast_print(print_start);
  print_start = "\n";
  first_print = true;
}
}  // namespace smart_io
template <class T>
ostream &operator,(ostream &os, const T &object) {
  if (!smart_io::first_print) {
    fast_print(smart_io::sep);
  } else {
    smart_io::first_print = false;
  }
  fast_print(object);
  return os;
}
template <class T>
istream &operator,(istream &is, T &object) {
  fast_scan(object);
  return is;
}
namespace typedefs {}
namespace numbers_operation {
template <class T>
T floor_mod(T a, T b) {
  if (a % b == 0) return 0;
  if (a >= 0 && b >= 0) return a % b;
  if (a <= 0 && b <= 0) return a % b;
  return abs(b) - (abs(a) % abs(b));
}
}  // namespace numbers_operation
using namespace numbers_operation;
using namespace typedefs;
int n;
vector<string> v;
bool expand(string &base, int i, string t) {
  for (int j = 0; j < ((int)t.size()); j++) {
    if (j + i >= ((int)base.size())) {
      base += t[j];
    } else {
      if (base[j + i] != t[j]) {
        return false;
      }
    }
  }
  return true;
}
signed main(signed argc, char *argv[]) {
  {
    cin, n;
    v.resize(n);
    cin, v;
  }
  {
    for (string s : v) {
      if (((int)set<char>(s.begin(), s.end()).size()) != ((int)s.size())) {
        smart_io::precall_print();
        cout, "NO";
        return 0;
      }
    }
  }
  {
    while (true) {
      vector<int> not_first_vec(26);
      vector<int> cnt(26);
      for (int i = 0; i < n; i++) {
        vector<int> local(26);
        for (int j = 0; j < ((int)v[i].size()); j++) {
          local[v[i][j] - 'a']++;
        }
        for (int j = 0; j < 26; j++) {
          if (local[j] > 1) {
            smart_io::precall_print();
            cout, "NO";
            return 0;
          }
        }
        cnt[v[i][0] - 'a']++;
        for (int j = 1; j < ((int)v[i].size()); j++) {
          not_first_vec[v[i][j] - 'a']++;
          cnt[v[i][j] - 'a']++;
        }
      }
      char c = '!';
      for (int i = 0; i < 26; i++) {
        if (not_first_vec[i] <= 1 && cnt[i] > 1) {
          c = i + 'a';
        }
      }
      if (c == '!') break;
      vector<pair<int, int>> where;
      for (int i = 0; i < n; i++) {
        for (int j = 0; j < ((int)v[i].size()); j++) {
          if (v[i][j] == c) {
            where.emplace_back(i, j);
          }
        }
      }
      if (where.empty()) continue;
      sort(where.begin(), where.end(), [](pair<int, int> a, pair<int, int> b) {
        return ((int)v[a.first].size()) < ((int)v[b.first].size());
      });
      int not_first = 0;
      pair<int, int> f = where[0];
      for (int i = 0; i < ((int)where.size()); i++) {
        if (where[i].second != 0) {
          not_first++;
          f = where[i];
        }
      }
      if (not_first > 1) {
        smart_io::precall_print();
        cout, "NO";
        return 0;
      }
      string base = v[f.first];
      for (int i = 0; i < ((int)where.size()); i++) {
        if (where[i].first == f.first) continue;
        if (!expand(base, f.second, v[where[i].first])) {
          smart_io::precall_print();
          cout, "NO";
          return 0;
        }
      }
      vector<bool> blocked(n, false);
      for (pair<int, int> p : where) {
        blocked[p.first] = true;
      }
      vector<string> _new;
      for (int i = 0; i < n; i++) {
        if (!blocked[i]) {
          _new.push_back(v[i]);
        }
      }
      _new.push_back(base);
      n = ((int)_new.size());
      v = _new;
      int tt = 0;
    }
  }
  {} {
    sort(v.begin(), v.end());
    string rez = "";
    for (string s : v) {
      rez += s;
    }
    if (((int)set<char>(rez.begin(), rez.end()).size()) == ((int)rez.size())) {
      smart_io::precall_print();
      cout, rez;
    } else {
      smart_io::precall_print();
      cout, "NO";
    }
  }
}
