#include <bits/stdc++.h>
using namespace std;
const int N = 26;
const long long M = 1e9 + 7;
long long n, k;
vector<int> f;
vector<string> s;
vector<int> nxt(N, -1);
vector<int> prv(N, -1);
bool zx[N];
int main() {
  cin >> n;
  s.resize(n);
  for (int i = 0; i < n; i++) {
    cin >> s[i];
    int l = s[i].length();
    vector<bool> qw(N, 0);
    for (int j = 0; j < l; j++) {
      int p = s[i][j] - 'a';
      if (qw[p]) {
        return cout << "NO", 0;
      }
      qw[p] = 1;
      zx[p] = 1;
      if (j != l - 1) {
        if (nxt[p] != s[i][j + 1] - 'a' && nxt[p] != -1) return cout << "NO", 0;
        nxt[p] = s[i][j + 1] - 'a';
      }
      if (j != 0) {
        if (prv[p] != s[i][j - 1] - 'a' && prv[p] != -1) return cout << "NO", 0;
        prv[p] = s[i][j - 1] - 'a';
      }
    }
  }
  string rez = "";
  for (int i = 0; i < N; i++) {
    if (prv[i] == -1 && zx[i]) {
      int y = i;
      while (y != -1) {
        rez += char(y + 'a');
        y = nxt[y];
      }
    }
  }
  int g = 0;
  for (int i = 0; i < N; i++)
    if (zx[i]) g++;
  if (rez.length() != g) return cout << "NO", 0;
  cout << rez;
  return 0;
}
