#include <bits/stdc++.h>
using namespace std;
int _n, n, i, j, k, h, x, gg, a[1111], L[1111], R[1111], in[1111], out[1111],
    vis[28], vs[28], vss[28];
string s[1111], _S;
vector<string> S;
vector<int> V[1111];
void ins(int x, int y) { V[x].push_back(y); }
void dfs(int x) {
  vis[x] = 1;
  _S.push_back('a' + x);
  vs[x] = 0;
  for (int i = 0, y; i < V[x].size(); i++)
    if (!vis[y = V[x][i]])
      dfs(y);
    else
      gg = 1;
}
int main() {
  scanf("%d", &n);
  if (n > 1000) return puts("NO"), 0;
  for (i = 1; i <= n; i++) {
    cin >> s[i];
    if (s[i].length() == 1) vs[s[i][0] - 'a'] = 1;
    vss[s[i][0] - 'a'] = 1;
    for (j = 1; j < s[i].length(); j++) {
      ins(s[i][j - 1] - 'a', s[i][j] - 'a');
      vss[s[i][j] - 'a'] = 1;
    }
  }
  for (i = 0; i < 26; i++)
    sort(V[i].begin(), V[i].end()),
        V[i].resize(unique(V[i].begin(), V[i].end()) - V[i].begin());
  for (i = 0; i < 26; i++)
    for (in[i] = V[i].size(), j = 0; j < V[i].size(); j++) out[V[i][j]]++;
  for (i = 0; i < 26; i++)
    if (V[i].size() > 1 || out[i] > 1) return puts("NO"), 0;
  for (i = 0; i < 26; i++)
    if (vss[i])
      if (in[i] && !out[i]) _S = "", dfs(i), S.push_back(_S);
  if (gg) return puts("NO"), 0;
  for (i = 0; i < 26; i++)
    if (!vis[i] && out[i]) return puts("NO"), 0;
  for (i = 0; i < 26; i++)
    if (!vis[i] && vs[i]) {
      _S = 'a' + i;
      S.push_back(_S);
    }
  sort(S.begin(), S.end());
  for (int i = 0; i < S.size(); i++) cout << S[i];
}
