#include <bits/stdc++.h>
const int MAX_N = 1e5 + 5;
int n, m;
char str[MAX_N], nxt[256];
bool chk[256], vis[256], not_root[256], flag;
std::vector<char> ans;
void dfs(char c) {
  if (!vis[c]) {
    vis[c] = true;
    ans.push_back(c);
    if (nxt[c]) dfs(nxt[c]);
  } else
    flag = true;
}
int main() {
  scanf("%d", &n);
  while (n--) {
    scanf(" %s", str);
    m = strlen(str);
    for (int i = 0; i < m; ++i) chk[str[i]] = true;
    for (int i = 0; i + 1 < m; ++i) {
      if (nxt[str[i]] == 0)
        nxt[str[i]] = str[i + 1];
      else if (nxt[str[i]] != str[i + 1]) {
        printf("NO\n");
        return 0;
      }
      not_root[str[i + 1]] = true;
    }
  }
  for (char i = 'a'; i <= 'z'; ++i)
    if (!not_root[i] && chk[i]) dfs(i);
  for (char i = 'a'; i <= 'z'; ++i) flag |= chk[i] != vis[i];
  if (flag) {
    printf("NO\n");
    return 0;
  }
  for (char c : ans) printf("%c", c);
  printf("\n");
  return 0;
}
