#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7, siz = 1e6 + 5;
long long t, n, m, k, in[30], out[30], a[siz];
bool vis[30], taken[siz], edg[30][30];
vector<long long> g[30];
string ans;
void go(long long u) {
  if (!taken[u]) ans += 'a' + u;
  vis[u] = 1;
  taken[u] = 1;
  for (auto x : g[u]) {
    if (vis[x]) {
      cout << "NO\n";
      exit(0);
    }
    go(x);
  }
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  vector<long long> st;
  for (long long i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (long long j = 0; j + 1 < s.size(); j++) {
      long long x = s[j] - 'a', y = s[j + 1] - 'a';
      if (!edg[x][y]) {
        edg[x][y] = 1;
        in[y]++;
        out[x]++;
        g[x].push_back(y);
      }
      if (in[y] > 1 or out[x] > 1) return cout << "NO\n", 0;
    }
    st.push_back(s[0] - 'a');
  }
  sort(st.begin(), st.end());
  for (auto x : st) {
    if (in[x] != 0) continue;
    memset(vis, 0, sizeof vis);
    go(x);
  }
  cout << ans << endl;
}
