#include <bits/stdc++.h>
using namespace std;
int prevc[26], nextc[26];
bool isE[26];
char s[100001];
bool read_data() {
  int n;
  memset(prevc, 0xff, sizeof(prevc));
  memset(nextc, 0xff, sizeof(nextc));
  memset(isE, 0, sizeof(isE));
  cin >> n;
  while (n--) {
    scanf(" %s", s);
    int len = strlen(s);
    for (int i = 0; i < len; i++) {
      int c = (s[i] - 'a');
      isE[c] = true;
      if (i != 0) {
        if (prevc[c] == -1 || prevc[c] == (s[i - 1] - 'a'))
          prevc[c] = (s[i - 1] - 'a');
        else
          return false;
      }
      if (i != len - 1) {
        if (nextc[c] == -1 || nextc[c] == (s[i + 1] - 'a'))
          nextc[c] = (s[i + 1] - 'a');
        else
          return false;
      }
    }
  }
  return true;
}
bool valid() {
  vector<string> s;
  s.assign(26, "");
  vector<int> isv;
  isv.assign(26, -1);
  queue<int> q;
  for (int i = 0; i < 26; i++)
    if (prevc[i] == -1 && isE[i]) {
      isv[i] = i;
      q.push(i);
      char ci = 'a' + i;
      s[i].push_back(ci);
    }
  while (!q.empty()) {
    int c = q.front();
    q.pop();
    int nc = nextc[c];
    if (nc == -1) continue;
    if (isv[nc] != -1) return false;
    isv[nc] = isv[c];
    q.push(nc);
    char nci = 'a' + nc;
    s[isv[c]].push_back(nci);
  }
  for (int i = 0; i < 26; i++)
    if (isE[i] & isv[i] == -1) return false;
  sort(s.begin(), s.end());
  for (int i = 0; i < 26; i++) cout << s[i];
  cout << endl;
  return true;
}
int main() {
  if (!read_data() || !valid()) {
    cout << "NO" << endl;
  }
  return 0;
}
