#include <bits/stdc++.h>
#pragma GCC optimize(1)
#pragma GCC optimize(2)
#pragma GCC optimize(3, "Ofast", "inline")
using namespace std;
inline long long read() {
  long long x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
inline void write(long long x) {
  if (x < 0) x = ~x + 1, putchar('-');
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
  puts("");
}
long long ksm(long long a, long long b) {
  long long res = 1;
  while (b) {
    if (b & 1) res = res * a;
    a = a * a;
    b >>= 1;
  }
  return res;
}
int g[26][26], a[26], in[26], out[26], vis[26];
string res;
int dfs(int u) {
  res = res + char(u + 'a');
  vis[u] = 1;
  for (int i = (0); i <= (25); i++)
    if (g[u][i]) {
      if (vis[i]) return 0;
      return dfs(i);
    }
  return 1;
}
int main() {
  int n = read();
  for (int i = (1); i <= (n); i++) {
    string s;
    cin >> s;
    for (int j = 0; j < s.size() - 1; j++) {
      g[s[j] - 'a'][s[j + 1] - 'a'] = 1;
    }
    if (s.size() == 1) a[s[0] - 'a'] = 1;
  }
  for (int i = (0); i <= (25); i++)
    for (int j = (0); j <= (25); j++)
      if (i == j && g[i][j]) {
        puts("NO");
        return 0;
      } else if (g[i][j])
        in[j]++, out[i]++;
  for (int i = (0); i <= (25); i++)
    if (in[i] > 1 || out[i] > 1) {
      puts("NO");
      return 0;
    }
  for (int i = (0); i <= (25); i++) {
    if (out[i] && !in[i]) {
      if (!dfs(i)) {
        puts("NO");
        return 0;
      }
    }
    if (a[i] && !out[i] && !in[i]) {
      res = res + char(i + 'a');
    }
  }
  for (int i = (0); i <= (25); i++) {
    if (in[i] && out[i] && !vis[i]) {
      puts("NO");
      return 0;
    }
  }
  cout << res << endl;
  return 0;
}
