#include <bits/stdc++.h>
using namespace std;
const int inf = 1000000007;
const long long linf = 1ll * inf * inf;
const int N = 100000 + 7;
const int M = 30;
const int multipleTest = 0;
int n;
string s[N];
int last[26];
int nxt[100];
bool valid[100];
bool has[100];
int cnt[100];
bool check[100];
void solve() {
  cin >> n;
  for (int j = (0), _b = (26); j < _b; ++j) nxt[j] = -1;
  for (int i = (0), _b = (n); i < _b; ++i) {
    cin >> s[i];
    for (int j = (0), _b = (26); j < _b; ++j) last[j] = -1;
    for (char c : s[i]) {
      has[c - 'a'] = true;
      if (last[c - 'a'] != -1) {
        puts("NO");
        return;
      }
      last[c - 'a'] = 0;
    }
    for (int j = (0), _b = ((int)(s[i]).size() - 1); j < _b; ++j) {
      int c = s[i][j] - 'a';
      int u = s[i][j + 1] - 'a';
      if (nxt[c] == -1 || nxt[c] == u) {
        nxt[c] = u;
        valid[u] = true;
      } else {
        puts("NO");
        return;
      }
    }
  }
  for (int j = (0), _b = (26); j < _b; ++j)
    if (has[j] && nxt[j] != -1) cnt[nxt[j]]++;
  for (int j = (0), _b = (26); j < _b; ++j)
    if (cnt[j] > 1) {
      puts("NO");
      return;
    }
  for (int j = (0), _b = (26); j < _b; ++j)
    if (has[j] && cnt[j] == 1) {
      int tmp = nxt[j];
      while (tmp != -1) {
        if (tmp == j) {
          puts("NO");
          return;
        }
        tmp = nxt[tmp];
      }
    }
  for (int j = (0), _b = (26); j < _b; ++j)
    if (has[j] && !valid[j]) {
      int tmp = j;
      while (tmp != -1) {
        printf("%c", char(tmp + 'a'));
        valid[tmp] = true;
        tmp = nxt[tmp];
      }
    }
}
int main() {
  int Test = 1;
  if (multipleTest) {
    cin >> Test;
  }
  for (int i = 0; i < Test; ++i) {
    solve();
  }
}
