#include <bits/stdc++.h>
using namespace std;
struct point {
  bool is;
  char val;
  char prev;
  char next;
};
int main() {
  int i, n, cnt;
  scanf("%d ", &n);
  map<char, point> v;
  for (i = 0; i < 26; i++) {
    v[i + 'a'].prev = 0;
    v[i + 'a'].next = 0;
  }
  string s;
  set<char> a;
  for (i = 0; i < n; i++) {
    cin >> s;
    a.clear();
    for (int j = 0; j < s.size(); j++) {
      if (a.count(s[j])) {
        printf("NO");
        return 0;
      } else
        a.insert(s[j]);
      if (v[s[j]].is) {
        if (j > 0) {
          if (v[s[j]].prev == 0) v[s[j]].prev = s[j - 1];
          if (v[s[j]].prev != s[j - 1]) {
            printf("NO");
            return 0;
          }
        }
        if (j < s.size() - 1) {
          if (v[s[j]].next == 0) v[s[j]].next = s[j + 1];
          if (v[s[j]].next != s[j + 1]) {
            printf("NO");
            return 0;
          }
        }
      }
      if (!v[s[j]].is) {
        v[s[j]].val = s[j];
        if (j > 0) {
          if (v[s[j]].prev == 0) v[s[j]].prev = s[j - 1];
          if (v[s[j]].prev != s[j - 1]) {
            printf("NO");
            return 0;
          }
        }
        if (j < s.size() - 1) {
          if (v[s[j]].next == 0) v[s[j]].next = s[j + 1];
          if (v[s[j]].next != s[j + 1]) {
            printf("NO");
            return 0;
          }
        }
        v[s[j]].is = 1;
      }
    }
  }
  vector<char> ans;
  cnt = 0;
  int cnt2 = 0;
  for (i = 0; i < 26; i++)
    if (v['a' + i].is) {
      if (v['a' + i].prev == 0) ans.push_back('a' + i);
      cnt2++;
    }
  if (ans.size() == 0) {
    printf("NO");
    return 0;
  }
  sort(ans.begin(), ans.end());
  for (i = 0; i < ans.size(); i++) {
    point p;
    cnt++;
    for (p = v[ans[i]]; p.next != 0; p = v[p.next]) cnt++;
  }
  if (cnt != cnt2) {
    printf("NO");
    return 0;
  }
  for (i = 0; i < ans.size(); i++) {
    point p;
    for (p = v[ans[i]]; p.next != 0; p = v[p.next]) printf("%c", p.val);
    printf("%c", p.val);
  }
  return 0;
}
