#include <bits/stdc++.h>
using namespace std;
const long double pi = acos(-1);
const int oo = 1 << 30;
const long long OO = 1e18;
const int N = 2e5 + 100;
int gi() {
  int w = 0;
  bool q = 1;
  char c = getchar();
  while ((c < '0' || c > '9') && c != '-') c = getchar();
  if (c == '-') q = 0, c = getchar();
  while (c >= '0' && c <= '9') w = w * 10 + c - '0', c = getchar();
  return q ? w : -w;
}
int f[N], g[N];
bool o[N];
string ans;
int sum;
inline void dfs(int k) {
  ans.push_back(k - 1 + 'a');
  sum++;
  if (f[k]) dfs(f[k]);
}
int main() {
  int n = gi(), i;
  string x;
  while (n--) {
    cin >> x;
    if (x.size() == 1)
      o[x[0] - 'a' + 1] = true;
    else
      for (i = 1; i < x.size(); i++) {
        if (!f[x[i - 1] - 'a' + 1])
          f[x[i - 1] - 'a' + 1] = x[i] - 'a' + 1;
        else if (f[x[i - 1] - 'a' + 1] != x[i] - 'a' + 1)
          return puts("NO"), 0;
        if (!g[x[i] - 'a' + 1])
          g[x[i] - 'a' + 1] = x[i - 1] - 'a' + 1;
        else if (g[x[i] - 'a' + 1] != x[i - 1] - 'a' + 1)
          return puts("NO"), 0;
      }
  }
  for (i = 1; i <= 26; i++) {
    if (!g[i] && (o[i] || f[i]))
      dfs(i);
    else
      sum += !g[i] && !f[i] && !o[i];
  }
  if (sum != 26)
    puts("NO");
  else
    cout << ans << endl;
  return 0;
}
