#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 100;
const int L = 26;
char s[maxn];
int nex[30], pre[30];
bool vis[30];
int main() {
  int n;
  for (; scanf("%d", &n) != EOF;) {
    for (int i = (0), _i = (L - 1); i <= _i; ++i) nex[i] = pre[i] = -1;
    for (int i = (0), _i = (L - 1); i <= _i; ++i) vis[i] = 0;
    bool flag = 1;
    for (int _ = (1), __ = (n); _ <= __; ++_) {
      scanf("%s", s);
      int len = strlen(s);
      for (int i = (0), _i = (len - 1); i <= _i; ++i) vis[s[i] - 'a'] = 1;
      for (int i = (1), _i = (len - 1); i <= _i; ++i) {
        int a = s[i - 1] - 'a', b = s[i] - 'a';
        if ((nex[a] != -1 && nex[a] != b) || (pre[b] != -1 && pre[b] != a)) {
          flag = 0;
        }
        nex[a] = b;
        pre[b] = a;
      }
    }
    if (!flag)
      puts("NO");
    else {
      string ans = "";
      for (int i = (0), _i = (L - 1); i <= _i; ++i)
        if (vis[i] && pre[i] == -1) {
          for (int x = i; x != -1; x = nex[x])
            vis[x] = 0, ans += (char)(x + 'a');
        }
      for (int i = (0), _i = (L - 1); i <= _i; ++i)
        if (vis[i]) {
          flag = 0;
          break;
        }
      if (!flag)
        puts("NO");
      else
        cout << ans << endl;
    }
  }
  return 0;
}
