#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
void solve() {
  int n;
  cin >> n;
  unordered_map<int, int> g;
  map<int, int> ins;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (int j = 1; j < s.size(); j++) {
      int u = s[j - 1] - 'a';
      int v = s[j] - 'a';
      if (g.find(u) != g.end()) {
        if (g[u] != v) {
          cout << "NO" << endl;
          return;
        }
      }
      if (ins.find(u) == ins.end()) ins[u] = 0;
      ins[v] = 1;
      g[u] = v;
    }
  }
  string ans = "";
  for (pair<int, int> it : ins) {
    if (it.second != 0) continue;
    unordered_set<int> visited;
    int u = it.first;
    ans += (u + 'a');
    visited.insert(u);
    while (g.find(u) != g.end()) {
      u = g[u];
      if (visited.find(u) != visited.end()) {
        cout << "NO" << endl;
        return;
      }
      ans += (u + 'a');
      visited.insert(u);
    }
  }
  cout << ans << endl;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  solve();
  return 0;
}
