#include <bits/stdc++.h>
using namespace std;
const long double pi = acosl(-1);
const int maxn = 200010;
const long long mod = 1e6 + 3;
const int inf = 1e9;
int n;
char s[maxn];
int a[26][26];
vector<int> g[26];
int vis[26], vis2[26], deg[26];
set<string> vec;
string str;
void die() {
  printf("NO\n");
  exit(0);
}
void dfs(int i) {
  if (vis2[i] || g[i].size() > 1) die();
  vis2[i] = 1;
  str += char(i + 'a');
  if (!g[i].empty()) dfs(g[i][0]);
}
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    scanf("%s", s);
    int len = strlen(s);
    for (int j = 0; j < len - 1; j++)
      a[s[j] - 'a'][s[j + 1] - 'a'] = 1, vis[s[j] - 'a'] = 1;
    vis[s[len - 1] - 'a'] = 1;
  }
  for (int i = 0; i < 26; i++)
    for (int j = 0; j < 26; j++) {
      if (a[i][j]) g[i].push_back(j), deg[j]++;
    }
  for (int i = 0; i < 26; i++) {
    if (!deg[i] && !g[i].empty()) {
      str = "";
      dfs(i);
      vec.insert(str);
    }
  }
  for (int i = 0; i < 26; i++) {
    if (vis[i] != vis2[i]) {
      if (deg[i])
        die();
      else
        vec.insert(string(1, char(i + 'a')));
    }
  }
  for (auto s : vec) cout << s;
  return 0;
}
